diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 33abcb2af1..7b85189b78 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -16227,7 +16227,7 @@ namespace ts {
                     checkTypeAssignableTo(staticType, getTypeWithoutSignatures(staticBaseType), node.name || node,
                         Diagnostics.Class_static_side_0_incorrectly_extends_base_class_static_side_1);
 
-                    if (baseType.symbol.valueDeclaration && !(baseType.symbol.valueDeclaration.flags & NodeFlags.Ambient)) {
+                    if (baseType.symbol.valueDeclaration && !isInAmbientContext(baseType.symbol.valueDeclaration)) {
                         if (!isBlockScopedNameDeclaredBeforeUse(baseType.symbol.valueDeclaration, node)) {
                             error(baseTypeNode, Diagnostics.A_class_must_be_declared_after_its_base_class);
                         }
diff --git a/tests/baselines/reference/ambientClassDeclaredBeforeBase.symbols b/tests/baselines/reference/ambientClassDeclaredBeforeBase.symbols
new file mode 100644
index 0000000000..0d9fed7724
--- /dev/null
+++ b/tests/baselines/reference/ambientClassDeclaredBeforeBase.symbols
@@ -0,0 +1,13 @@
+=== tests/cases/compiler/a.d.ts ===
+
+declare namespace ns {
+>ns : Symbol(ns, Decl(a.d.ts, 0, 0))
+
+  class SecondNS extends FirstNS { }
+>SecondNS : Symbol(SecondNS, Decl(a.d.ts, 1, 22))
+>FirstNS : Symbol(FirstNS, Decl(a.d.ts, 2, 36))
+
+  class FirstNS { }
+>FirstNS : Symbol(FirstNS, Decl(a.d.ts, 2, 36))
+}
+
diff --git a/tests/baselines/reference/ambientClassDeclaredBeforeBase.types b/tests/baselines/reference/ambientClassDeclaredBeforeBase.types
new file mode 100644
index 0000000000..554d3cfcce
--- /dev/null
+++ b/tests/baselines/reference/ambientClassDeclaredBeforeBase.types
@@ -0,0 +1,13 @@
+=== tests/cases/compiler/a.d.ts ===
+
+declare namespace ns {
+>ns : typeof ns
+
+  class SecondNS extends FirstNS { }
+>SecondNS : SecondNS
+>FirstNS : FirstNS
+
+  class FirstNS { }
+>FirstNS : FirstNS
+}
+
diff --git a/tests/cases/compiler/ambientClassDeclaredBeforeBase.ts b/tests/cases/compiler/ambientClassDeclaredBeforeBase.ts
new file mode 100644
index 0000000000..7e564c9d37
--- /dev/null
+++ b/tests/cases/compiler/ambientClassDeclaredBeforeBase.ts
@@ -0,0 +1,6 @@
+// @filename: a.d.ts
+
+declare namespace ns {
+  class SecondNS extends FirstNS { }
+  class FirstNS { }
+}