diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 21ae2369a4..cb373b4375 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -15221,7 +15221,7 @@ namespace ts {
                     if (indexInfo.isReadonly && (isAssignmentTarget(node) || isDeleteTarget(node))) {
                         error(node, Diagnostics.Index_signature_in_type_0_only_permits_reading, typeToString(apparentType));
                     }
-                    return indexInfo.type;
+                    return getFlowTypeOfPropertyAccess(node, /*prop*/ undefined, indexInfo.type, getAssignmentTargetKind(node));
                 }
                 if (right.escapedText && !checkAndReportErrorForExtendingInterface(node)) {
                     reportNonexistentProperty(right, type.flags & TypeFlags.TypeParameter && (type as TypeParameter).isThisType ? apparentType : type);
@@ -15246,16 +15246,21 @@ namespace ts {
                     return unknownType;
                 }
             }
+            return getFlowTypeOfPropertyAccess(node, prop, propType, assignmentKind);
+        }
 
-            // Only compute control flow type if this is a property access expression that isn't an
-            // assignment target, and the referenced property was declared as a variable, property,
-            // accessor, or optional method.
-            if (node.kind !== SyntaxKind.PropertyAccessExpression || assignmentKind === AssignmentKind.Definite ||
-                !(prop.flags & (SymbolFlags.Variable | SymbolFlags.Property | SymbolFlags.Accessor)) &&
-                !(prop.flags & SymbolFlags.Method && propType.flags & TypeFlags.Union)) {
-                return propType;
+        /**
+         * Only compute control flow type if this is a property access expression that isn't an
+         * assignment target, and the referenced property was declared as a variable, property,
+         * accessor, or optional method.
+         */
+        function getFlowTypeOfPropertyAccess(node: PropertyAccessExpression | QualifiedName, prop: Symbol | undefined, type: Type, assignmentKind: AssignmentKind) {
+            if (node.kind !== SyntaxKind.PropertyAccessExpression ||
+                assignmentKind === AssignmentKind.Definite ||
+                prop && !(prop.flags & (SymbolFlags.Variable | SymbolFlags.Property | SymbolFlags.Accessor)) && !(prop.flags & SymbolFlags.Method && type.flags & TypeFlags.Union)) {
+                return type;
             }
-            const flowType = getFlowTypeOfReference(node, propType);
+            const flowType = getFlowTypeOfReference(node, type);
             return assignmentKind ? getBaseTypeOfLiteralType(flowType) : flowType;
         }
 
diff --git a/tests/baselines/reference/controlFlowStringIndex.js b/tests/baselines/reference/controlFlowStringIndex.js
new file mode 100644
index 0000000000..db4d4a30ab
--- /dev/null
+++ b/tests/baselines/reference/controlFlowStringIndex.js
@@ -0,0 +1,20 @@
+//// [controlFlowStringIndex.ts]
+type A = {
+    other: number | null;
+    [index: string]: number | null
+};
+declare const value: A;
+if (value.foo !== null) {
+    value.foo.toExponential()
+    value.other // should still be number | null
+    value.bar // should still be number | null
+}
+
+
+//// [controlFlowStringIndex.js]
+"use strict";
+if (value.foo !== null) {
+    value.foo.toExponential();
+    value.other; // should still be number | null
+    value.bar; // should still be number | null
+}
diff --git a/tests/baselines/reference/controlFlowStringIndex.symbols b/tests/baselines/reference/controlFlowStringIndex.symbols
new file mode 100644
index 0000000000..5c7d626d4a
--- /dev/null
+++ b/tests/baselines/reference/controlFlowStringIndex.symbols
@@ -0,0 +1,32 @@
+=== tests/cases/conformance/controlFlow/controlFlowStringIndex.ts ===
+type A = {
+>A : Symbol(A, Decl(controlFlowStringIndex.ts, 0, 0))
+
+    other: number | null;
+>other : Symbol(other, Decl(controlFlowStringIndex.ts, 0, 10))
+
+    [index: string]: number | null
+>index : Symbol(index, Decl(controlFlowStringIndex.ts, 2, 5))
+
+};
+declare const value: A;
+>value : Symbol(value, Decl(controlFlowStringIndex.ts, 4, 13))
+>A : Symbol(A, Decl(controlFlowStringIndex.ts, 0, 0))
+
+if (value.foo !== null) {
+>value : Symbol(value, Decl(controlFlowStringIndex.ts, 4, 13))
+
+    value.foo.toExponential()
+>value.foo.toExponential : Symbol(Number.toExponential, Decl(lib.d.ts, --, --))
+>value : Symbol(value, Decl(controlFlowStringIndex.ts, 4, 13))
+>toExponential : Symbol(Number.toExponential, Decl(lib.d.ts, --, --))
+
+    value.other // should still be number | null
+>value.other : Symbol(other, Decl(controlFlowStringIndex.ts, 0, 10))
+>value : Symbol(value, Decl(controlFlowStringIndex.ts, 4, 13))
+>other : Symbol(other, Decl(controlFlowStringIndex.ts, 0, 10))
+
+    value.bar // should still be number | null
+>value : Symbol(value, Decl(controlFlowStringIndex.ts, 4, 13))
+}
+
diff --git a/tests/baselines/reference/controlFlowStringIndex.types b/tests/baselines/reference/controlFlowStringIndex.types
new file mode 100644
index 0000000000..8c5f82cbf8
--- /dev/null
+++ b/tests/baselines/reference/controlFlowStringIndex.types
@@ -0,0 +1,43 @@
+=== tests/cases/conformance/controlFlow/controlFlowStringIndex.ts ===
+type A = {
+>A : A
+
+    other: number | null;
+>other : number | null
+>null : null
+
+    [index: string]: number | null
+>index : string
+>null : null
+
+};
+declare const value: A;
+>value : A
+>A : A
+
+if (value.foo !== null) {
+>value.foo !== null : boolean
+>value.foo : number | null
+>value : A
+>foo : number | null
+>null : null
+
+    value.foo.toExponential()
+>value.foo.toExponential() : string
+>value.foo.toExponential : (fractionDigits?: number | undefined) => string
+>value.foo : number
+>value : A
+>foo : number
+>toExponential : (fractionDigits?: number | undefined) => string
+
+    value.other // should still be number | null
+>value.other : number | null
+>value : A
+>other : number | null
+
+    value.bar // should still be number | null
+>value.bar : number | null
+>value : A
+>bar : number | null
+}
+
diff --git a/tests/cases/conformance/controlFlow/controlFlowStringIndex.ts b/tests/cases/conformance/controlFlow/controlFlowStringIndex.ts
new file mode 100644
index 0000000000..28b75eaba7
--- /dev/null
+++ b/tests/cases/conformance/controlFlow/controlFlowStringIndex.ts
@@ -0,0 +1,11 @@
+// @strict: true
+type A = {
+    other: number | null;
+    [index: string]: number | null
+};
+declare const value: A;
+if (value.foo !== null) {
+    value.foo.toExponential()
+    value.other // should still be number | null
+    value.bar // should still be number | null
+}