diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 148345c8ef..bd48701de4 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -9806,7 +9806,7 @@ namespace ts {
          * this function should be called in a left folding style, with left = previous result of getSpreadType
          * and right = the new element to be spread.
          */
-        function getSpreadType(left: Type, right: Type, symbol: Symbol | undefined, typeFlags: TypeFlags, objectFlags: ObjectFlags): Type {
+        function getSpreadType(left: Type, right: Type, symbol: Symbol | undefined, typeFlags: TypeFlags, objectFlags: ObjectFlags, isUnion?: boolean): Type {
             if (left.flags & TypeFlags.Any || right.flags & TypeFlags.Any) {
                 return anyType;
             }
@@ -9820,10 +9820,10 @@ namespace ts {
                 return left;
             }
             if (left.flags & TypeFlags.Union) {
-                return mapType(left, t => getSpreadType(t, right, symbol, typeFlags, objectFlags));
+                return mapType(left, t => getSpreadType(t, right, symbol, typeFlags, objectFlags, isUnion));
             }
             if (right.flags & TypeFlags.Union) {
-                return mapType(right, t => getSpreadType(left, t, symbol, typeFlags, objectFlags));
+                return mapType(right, t => getSpreadType(left, t, symbol, typeFlags, objectFlags, true));
             }
             if (right.flags & (TypeFlags.BooleanLike | TypeFlags.NumberLike | TypeFlags.StringLike | TypeFlags.EnumLike | TypeFlags.NonPrimitive | TypeFlags.Index)) {
                 return left;
@@ -9870,7 +9870,12 @@ namespace ts {
                         result.nameType = leftProp.nameType;
                         members.set(leftProp.escapedName, result);
                     }
-                    else if (symbol && shouldCheckAsExcessProperty(leftProp, symbol) && !shouldCheckAsExcessProperty(rightProp, symbol)) {
+                    else if (strictNullChecks &&
+                             !isUnion &&
+                             symbol &&
+                             shouldCheckAsExcessProperty(leftProp, symbol) &&
+                             !shouldCheckAsExcessProperty(rightProp, symbol) &&
+                             !(getFalsyFlags(rightType) & TypeFlags.Nullable)) {
                         error(leftProp.valueDeclaration, Diagnostics._0_is_overwritten_by_a_later_spread, unescapeLeadingUnderscores(leftProp.escapedName));
                     }
                 }
diff --git a/tests/baselines/reference/checkJsxChildrenProperty13.errors.txt b/tests/baselines/reference/checkJsxChildrenProperty13.errors.txt
deleted file mode 100644
index d375a6e0c0..0000000000
--- a/tests/baselines/reference/checkJsxChildrenProperty13.errors.txt
+++ /dev/null
@@ -1,33 +0,0 @@
-tests/cases/conformance/jsx/file.tsx(12,46): error TS2735: 'children' is overwritten by a later spread.
-
-
-==== tests/cases/conformance/jsx/file.tsx (1 errors) ====
-    import React = require('react');
-    
-    interface ButtonProp {
-        a: number,
-        b: string,
-        children: Button;
-    }
-    
-    class Button extends React.Component<ButtonProp, any> {
-        render() {
-            // Error children are specified twice
-            return (<InnerButton {...this.props} children="hi">
-                                                 ~~~~~~~~~~~~~
-!!! error TS2735: 'children' is overwritten by a later spread.
-                <div>Hello World</div>
-                </InnerButton>);
-        }
-    }
-    
-    interface InnerButtonProp {
-    	a: number
-    }
-    
-    class InnerButton extends React.Component<InnerButtonProp, any> {
-    	render() {
-    		return (<button>Hello</button>);
-    	}
-    }
-    
\ No newline at end of file
diff --git a/tests/baselines/reference/checkJsxChildrenProperty2.errors.txt b/tests/baselines/reference/checkJsxChildrenProperty2.errors.txt
index 38f6fae244..0282c566e4 100644
--- a/tests/baselines/reference/checkJsxChildrenProperty2.errors.txt
+++ b/tests/baselines/reference/checkJsxChildrenProperty2.errors.txt
@@ -1,6 +1,5 @@
 tests/cases/conformance/jsx/file.tsx(14,10): error TS2322: Type '{ a: number; b: string; }' is not assignable to type 'Prop'.
   Property 'children' is missing in type '{ a: number; b: string; }'.
-tests/cases/conformance/jsx/file.tsx(17,25): error TS2735: 'children' is overwritten by a later spread.
 tests/cases/conformance/jsx/file.tsx(31,6): error TS2322: Type '{ children: (Element | ((name: string) => Element))[]; a: number; b: string; }' is not assignable to type 'Prop'.
   Types of property 'children' are incompatible.
     Type '(Element | ((name: string) => Element))[]' is not assignable to type 'string | Element'.
@@ -23,7 +22,7 @@ tests/cases/conformance/jsx/file.tsx(49,6): error TS2322: Type '{ children: Elem
         Property 'type' is missing in type 'Element[]'.
 
 
-==== tests/cases/conformance/jsx/file.tsx (6 errors) ====
+==== tests/cases/conformance/jsx/file.tsx (5 errors) ====
     import React = require('react');
     
     interface Prop {
@@ -44,8 +43,6 @@ tests/cases/conformance/jsx/file.tsx(49,6): error TS2322: Type '{ children: Elem
     
     let k0 =
         <Comp a={10} b="hi" children="Random" >
-                            ~~~~~~~~~~~~~~~~~
-!!! error TS2735: 'children' is overwritten by a later spread.
             hi hi hi!
         </Comp>;
     
diff --git a/tests/baselines/reference/objectSpreadNegative.errors.txt b/tests/baselines/reference/objectSpreadNegative.errors.txt
index 57de2e1797..0d6a58fe76 100644
--- a/tests/baselines/reference/objectSpreadNegative.errors.txt
+++ b/tests/baselines/reference/objectSpreadNegative.errors.txt
@@ -5,15 +5,8 @@ tests/cases/conformance/types/spread/objectSpreadNegative.ts(23,1): error TS2322
   Property 'b' is missing in type '{ s: string; }'.
 tests/cases/conformance/types/spread/objectSpreadNegative.ts(25,1): error TS2322: Type '{ b: boolean; }' is not assignable to type '{ s: string; b: boolean; }'.
   Property 's' is missing in type '{ b: boolean; }'.
-tests/cases/conformance/types/spread/objectSpreadNegative.ts(27,20): error TS2735: 'b' is overwritten by a later spread.
 tests/cases/conformance/types/spread/objectSpreadNegative.ts(27,36): error TS2300: Duplicate identifier 'b'.
 tests/cases/conformance/types/spread/objectSpreadNegative.ts(27,53): error TS2300: Duplicate identifier 'b'.
-tests/cases/conformance/types/spread/objectSpreadNegative.ts(31,7): error TS2735: 'b' is overwritten by a later spread.
-tests/cases/conformance/types/spread/objectSpreadNegative.ts(36,7): error TS2735: 'b' is overwritten by a later spread.
-tests/cases/conformance/types/spread/objectSpreadNegative.ts(38,14): error TS2735: 'b' is overwritten by a later spread.
-tests/cases/conformance/types/spread/objectSpreadNegative.ts(40,53): error TS2735: 'd' is overwritten by a later spread.
-tests/cases/conformance/types/spread/objectSpreadNegative.ts(42,7): error TS2735: 'a' is overwritten by a later spread.
-tests/cases/conformance/types/spread/objectSpreadNegative.ts(44,37): error TS2735: 'b' is overwritten by a later spread.
 tests/cases/conformance/types/spread/objectSpreadNegative.ts(47,19): error TS2698: Spread types may only be created from object types.
 tests/cases/conformance/types/spread/objectSpreadNegative.ts(48,19): error TS2698: Spread types may only be created from object types.
 tests/cases/conformance/types/spread/objectSpreadNegative.ts(49,20): error TS2698: Spread types may only be created from object types.
@@ -27,7 +20,7 @@ tests/cases/conformance/types/spread/objectSpreadNegative.ts(77,14): error TS269
 tests/cases/conformance/types/spread/objectSpreadNegative.ts(80,14): error TS2698: Spread types may only be created from object types.
 
 
-==== tests/cases/conformance/types/spread/objectSpreadNegative.ts (24 errors) ====
+==== tests/cases/conformance/types/spread/objectSpreadNegative.ts (17 errors) ====
     let o = { a: 1, b: 'no' }
     
     /// private propagates
@@ -66,8 +59,6 @@ tests/cases/conformance/types/spread/objectSpreadNegative.ts(80,14): error TS269
 !!! error TS2322:   Property 's' is missing in type '{ b: boolean; }'.
     
     let duplicated = { b: 'bad', ...o, b: 'bad', ...o2, b: 'bad' }
-                       ~~~~~~~~
-!!! error TS2735: 'b' is overwritten by a later spread.
                                        ~
 !!! error TS2300: Duplicate identifier 'b'.
                                                         ~
@@ -76,31 +67,19 @@ tests/cases/conformance/types/spread/objectSpreadNegative.ts(80,14): error TS269
     // Note: ignore changes the order that properties are printed
     let ignore: { a: number, b: string } =
         { b: 'ignored', ...o }
-          ~~~~~~~~~~~~
-!!! error TS2735: 'b' is overwritten by a later spread.
     
     let o3 = { a: 1, b: 'no' }
     let o4 = { b: 'yes', c: true }
     let combinedBefore: { a: number, b: string, c: boolean } =
         { b: 'ok', ...o3, ...o4 }
-          ~~~~~~~
-!!! error TS2735: 'b' is overwritten by a later spread.
     let combinedMid: { a: number, b: string, c: boolean } =
         { ...o3, b: 'ok', ...o4 }
-                 ~~~~~~~
-!!! error TS2735: 'b' is overwritten by a later spread.
     let combinedNested: { a: number, b: boolean, c: string, d: string } =
         { ...{ a: 4, ...{ b: false, c: 'overriden' } }, d: 'actually new', ...{ a: 5, d: 'maybe new' } }
-                                                        ~~~~~~~~~~~~~~~~~
-!!! error TS2735: 'd' is overwritten by a later spread.
     let changeTypeBefore: { a: number, b: string } =
         { a: 'wrong type?', ...o3 };
-          ~~~~~~~~~~~~~~~~
-!!! error TS2735: 'a' is overwritten by a later spread.
     let computedMiddle: { a: number, b: string, c: boolean, "in the middle": number } =
         { ...o3, ['in the middle']: 13, b: 'maybe?', ...o4 }
-                                        ~~~~~~~~~~~
-!!! error TS2735: 'b' is overwritten by a later spread.
     
     // primitives are not allowed, except for falsy ones
     let spreadNum = { ...12 };
diff --git a/tests/baselines/reference/spreadOverwritesProperty.errors.txt b/tests/baselines/reference/spreadOverwritesProperty.errors.txt
deleted file mode 100644
index a6b78900b4..0000000000
--- a/tests/baselines/reference/spreadOverwritesProperty.errors.txt
+++ /dev/null
@@ -1,12 +0,0 @@
-tests/cases/conformance/types/spread/spreadOverwritesProperty.ts(3,17): error TS2735: 'b' is overwritten by a later spread.
-
-
-==== tests/cases/conformance/types/spread/spreadOverwritesProperty.ts (1 errors) ====
-    declare var ab: { a: number, b: number };
-    declare var abq: { a: number, b?: number };
-    var unused1 = { b: 1, ...ab }
-                    ~~~~
-!!! error TS2735: 'b' is overwritten by a later spread.
-    var unused2 = { ...ab, ...ab }
-    var unused3 = { b: 1, ...abq }
-    
\ No newline at end of file
diff --git a/tests/baselines/reference/spreadOverwritesProperty.js b/tests/baselines/reference/spreadOverwritesProperty.js
index ac28594c4b..653bc11367 100644
--- a/tests/baselines/reference/spreadOverwritesProperty.js
+++ b/tests/baselines/reference/spreadOverwritesProperty.js
@@ -4,6 +4,13 @@ declare var abq: { a: number, b?: number };
 var unused1 = { b: 1, ...ab }
 var unused2 = { ...ab, ...ab }
 var unused3 = { b: 1, ...abq }
+
+function g(obj: { x: number | undefined }) {
+    return { x: 1, ...obj }; // should be allowed because of undefined
+}
+function h(obj: { x: number }) {
+    return { x: 1, ...obj }; // should be allowed because we don't know about undefined
+}
 
 
 //// [spreadOverwritesProperty.js]
@@ -21,3 +28,9 @@ var __assign = (this && this.__assign) || function () {
 var unused1 = __assign({ b: 1 }, ab);
 var unused2 = __assign({}, ab, ab);
 var unused3 = __assign({ b: 1 }, abq);
+function g(obj) {
+    return __assign({ x: 1 }, obj); // should be allowed because of undefined
+}
+function h(obj) {
+    return __assign({ x: 1 }, obj); // should be allowed because we don't know about undefined
+}
diff --git a/tests/baselines/reference/spreadOverwritesProperty.symbols b/tests/baselines/reference/spreadOverwritesProperty.symbols
index 599a40c855..7af75e14d3 100644
--- a/tests/baselines/reference/spreadOverwritesProperty.symbols
+++ b/tests/baselines/reference/spreadOverwritesProperty.symbols
@@ -24,3 +24,22 @@ var unused3 = { b: 1, ...abq }
 >b : Symbol(b, Decl(spreadOverwritesProperty.ts, 4, 15))
 >abq : Symbol(abq, Decl(spreadOverwritesProperty.ts, 1, 11))
 
+function g(obj: { x: number | undefined }) {
+>g : Symbol(g, Decl(spreadOverwritesProperty.ts, 4, 30))
+>obj : Symbol(obj, Decl(spreadOverwritesProperty.ts, 6, 11))
+>x : Symbol(x, Decl(spreadOverwritesProperty.ts, 6, 17))
+
+    return { x: 1, ...obj }; // should be allowed because of undefined
+>x : Symbol(x, Decl(spreadOverwritesProperty.ts, 7, 12))
+>obj : Symbol(obj, Decl(spreadOverwritesProperty.ts, 6, 11))
+}
+function h(obj: { x: number }) {
+>h : Symbol(h, Decl(spreadOverwritesProperty.ts, 8, 1))
+>obj : Symbol(obj, Decl(spreadOverwritesProperty.ts, 9, 11))
+>x : Symbol(x, Decl(spreadOverwritesProperty.ts, 9, 17))
+
+    return { x: 1, ...obj }; // should be allowed because we don't know about undefined
+>x : Symbol(x, Decl(spreadOverwritesProperty.ts, 10, 12))
+>obj : Symbol(obj, Decl(spreadOverwritesProperty.ts, 9, 11))
+}
+
diff --git a/tests/baselines/reference/spreadOverwritesProperty.types b/tests/baselines/reference/spreadOverwritesProperty.types
index 9d6630a406..0dad333c3a 100644
--- a/tests/baselines/reference/spreadOverwritesProperty.types
+++ b/tests/baselines/reference/spreadOverwritesProperty.types
@@ -29,3 +29,26 @@ var unused3 = { b: 1, ...abq }
 >1 : 1
 >abq : { a: number; b?: number; }
 
+function g(obj: { x: number | undefined }) {
+>g : (obj: { x: number; }) => { x: number; }
+>obj : { x: number; }
+>x : number
+
+    return { x: 1, ...obj }; // should be allowed because of undefined
+>{ x: 1, ...obj } : { x: number; }
+>x : number
+>1 : 1
+>obj : { x: number; }
+}
+function h(obj: { x: number }) {
+>h : (obj: { x: number; }) => { x: number; }
+>obj : { x: number; }
+>x : number
+
+    return { x: 1, ...obj }; // should be allowed because we don't know about undefined
+>{ x: 1, ...obj } : { x: number; }
+>x : number
+>1 : 1
+>obj : { x: number; }
+}
+
diff --git a/tests/baselines/reference/spreadOverwritesPropertyStrict.errors.txt b/tests/baselines/reference/spreadOverwritesPropertyStrict.errors.txt
index 5a3706479a..70febf0086 100644
--- a/tests/baselines/reference/spreadOverwritesPropertyStrict.errors.txt
+++ b/tests/baselines/reference/spreadOverwritesPropertyStrict.errors.txt
@@ -9,4 +9,10 @@ tests/cases/conformance/types/spread/spreadOverwritesPropertyStrict.ts(3,17): er
 !!! error TS2735: 'b' is overwritten by a later spread.
     var unused2 = { ...ab, ...ab }
     var unused3 = { b: 1, ...abq }
+    function g(obj: { x: number | undefined }) {
+        return { x: 1, ...obj }; // should be allowed because of undefined
+    }
+    function f(obj: { x: number } | undefined) {
+        return { x: 1, ...obj };
+    }
     
\ No newline at end of file
diff --git a/tests/baselines/reference/spreadOverwritesPropertyStrict.js b/tests/baselines/reference/spreadOverwritesPropertyStrict.js
index 0767afcb58..457bc5ed7e 100644
--- a/tests/baselines/reference/spreadOverwritesPropertyStrict.js
+++ b/tests/baselines/reference/spreadOverwritesPropertyStrict.js
@@ -4,6 +4,12 @@ declare var abq: { a: number, b?: number };
 var unused1 = { b: 1, ...ab }
 var unused2 = { ...ab, ...ab }
 var unused3 = { b: 1, ...abq }
+function g(obj: { x: number | undefined }) {
+    return { x: 1, ...obj }; // should be allowed because of undefined
+}
+function f(obj: { x: number } | undefined) {
+    return { x: 1, ...obj };
+}
 
 
 //// [spreadOverwritesPropertyStrict.js]
@@ -22,3 +28,9 @@ var __assign = (this && this.__assign) || function () {
 var unused1 = __assign({ b: 1 }, ab);
 var unused2 = __assign({}, ab, ab);
 var unused3 = __assign({ b: 1 }, abq);
+function g(obj) {
+    return __assign({ x: 1 }, obj); // should be allowed because of undefined
+}
+function f(obj) {
+    return __assign({ x: 1 }, obj);
+}
diff --git a/tests/baselines/reference/spreadOverwritesPropertyStrict.symbols b/tests/baselines/reference/spreadOverwritesPropertyStrict.symbols
index 0f03d28fdc..0168d14296 100644
--- a/tests/baselines/reference/spreadOverwritesPropertyStrict.symbols
+++ b/tests/baselines/reference/spreadOverwritesPropertyStrict.symbols
@@ -24,3 +24,22 @@ var unused3 = { b: 1, ...abq }
 >b : Symbol(b, Decl(spreadOverwritesPropertyStrict.ts, 4, 15))
 >abq : Symbol(abq, Decl(spreadOverwritesPropertyStrict.ts, 1, 11))
 
+function g(obj: { x: number | undefined }) {
+>g : Symbol(g, Decl(spreadOverwritesPropertyStrict.ts, 4, 30))
+>obj : Symbol(obj, Decl(spreadOverwritesPropertyStrict.ts, 5, 11))
+>x : Symbol(x, Decl(spreadOverwritesPropertyStrict.ts, 5, 17))
+
+    return { x: 1, ...obj }; // should be allowed because of undefined
+>x : Symbol(x, Decl(spreadOverwritesPropertyStrict.ts, 6, 12))
+>obj : Symbol(obj, Decl(spreadOverwritesPropertyStrict.ts, 5, 11))
+}
+function f(obj: { x: number } | undefined) {
+>f : Symbol(f, Decl(spreadOverwritesPropertyStrict.ts, 7, 1))
+>obj : Symbol(obj, Decl(spreadOverwritesPropertyStrict.ts, 8, 11))
+>x : Symbol(x, Decl(spreadOverwritesPropertyStrict.ts, 8, 17))
+
+    return { x: 1, ...obj };
+>x : Symbol(x, Decl(spreadOverwritesPropertyStrict.ts, 9, 12))
+>obj : Symbol(obj, Decl(spreadOverwritesPropertyStrict.ts, 8, 11))
+}
+
diff --git a/tests/baselines/reference/spreadOverwritesPropertyStrict.types b/tests/baselines/reference/spreadOverwritesPropertyStrict.types
index f5a5492727..64350c3985 100644
--- a/tests/baselines/reference/spreadOverwritesPropertyStrict.types
+++ b/tests/baselines/reference/spreadOverwritesPropertyStrict.types
@@ -29,3 +29,26 @@ var unused3 = { b: 1, ...abq }
 >1 : 1
 >abq : { a: number; b?: number | undefined; }
 
+function g(obj: { x: number | undefined }) {
+>g : (obj: { x: number | undefined; }) => { x: number | undefined; }
+>obj : { x: number | undefined; }
+>x : number | undefined
+
+    return { x: 1, ...obj }; // should be allowed because of undefined
+>{ x: 1, ...obj } : { x: number | undefined; }
+>x : number
+>1 : 1
+>obj : { x: number | undefined; }
+}
+function f(obj: { x: number } | undefined) {
+>f : (obj: { x: number; } | undefined) => { x: number; } | { x: number; }
+>obj : { x: number; } | undefined
+>x : number
+
+    return { x: 1, ...obj };
+>{ x: 1, ...obj } : { x: number; } | { x: number; }
+>x : number
+>1 : 1
+>obj : { x: number; } | undefined
+}
+
diff --git a/tests/baselines/reference/spreadUnion3.errors.txt b/tests/baselines/reference/spreadUnion3.errors.txt
index b9c7c186fe..88aa12c708 100644
--- a/tests/baselines/reference/spreadUnion3.errors.txt
+++ b/tests/baselines/reference/spreadUnion3.errors.txt
@@ -1,18 +1,15 @@
 tests/cases/conformance/types/spread/spreadUnion3.ts(2,14): error TS2322: Type 'number' is not assignable to type 'string'.
-tests/cases/conformance/types/spread/spreadUnion3.ts(2,14): error TS2735: 'y' is overwritten by a later spread.
 tests/cases/conformance/types/spread/spreadUnion3.ts(9,23): error TS2339: Property 'a' does not exist on type '{}'.
 tests/cases/conformance/types/spread/spreadUnion3.ts(17,11): error TS2698: Spread types may only be created from object types.
 tests/cases/conformance/types/spread/spreadUnion3.ts(18,11): error TS2698: Spread types may only be created from object types.
 
 
-==== tests/cases/conformance/types/spread/spreadUnion3.ts (5 errors) ====
+==== tests/cases/conformance/types/spread/spreadUnion3.ts (4 errors) ====
     function f(x: { y: string } | undefined): { y: string } {
         return { y: 123, ...x } // y: string | number
                  ~
 !!! error TS2322: Type 'number' is not assignable to type 'string'.
 !!! related TS6500 tests/cases/conformance/types/spread/spreadUnion3.ts:1:45: The expected type comes from property 'y' which is declared here on type '{ y: string; }'
-                 ~~~~~~
-!!! error TS2735: 'y' is overwritten by a later spread.
     }
     f(undefined)
     
diff --git a/tests/cases/conformance/types/spread/spreadOverwritesProperty.ts b/tests/cases/conformance/types/spread/spreadOverwritesProperty.ts
index e4407fd87e..4ebc7f9fa0 100644
--- a/tests/cases/conformance/types/spread/spreadOverwritesProperty.ts
+++ b/tests/cases/conformance/types/spread/spreadOverwritesProperty.ts
@@ -3,3 +3,10 @@ declare var abq: { a: number, b?: number };
 var unused1 = { b: 1, ...ab }
 var unused2 = { ...ab, ...ab }
 var unused3 = { b: 1, ...abq }
+
+function g(obj: { x: number | undefined }) {
+    return { x: 1, ...obj }; // should be allowed because of undefined
+}
+function h(obj: { x: number }) {
+    return { x: 1, ...obj }; // should be allowed because we don't know about undefined
+}
diff --git a/tests/cases/conformance/types/spread/spreadOverwritesPropertyStrict.ts b/tests/cases/conformance/types/spread/spreadOverwritesPropertyStrict.ts
index d1df11e49b..e2bf6da240 100644
--- a/tests/cases/conformance/types/spread/spreadOverwritesPropertyStrict.ts
+++ b/tests/cases/conformance/types/spread/spreadOverwritesPropertyStrict.ts
@@ -4,3 +4,9 @@ declare var abq: { a: number, b?: number };
 var unused1 = { b: 1, ...ab }
 var unused2 = { ...ab, ...ab }
 var unused3 = { b: 1, ...abq }
+function g(obj: { x: number | undefined }) {
+    return { x: 1, ...obj }; // should be allowed because of undefined
+}
+function f(obj: { x: number } | undefined) {
+    return { x: 1, ...obj };
+}