diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index a93189c81f2..c7cdb922cc6 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -8781,7 +8781,7 @@ namespace ts {
         }
 
         function isMappableType(type: Type) {
-            return type.flags & (TypeFlags.Any | TypeFlags.TypeParameter | TypeFlags.Object | TypeFlags.Intersection | TypeFlags.IndexedAccess);
+            return type.flags & (TypeFlags.Any | TypeFlags.InstantiableNonPrimitive | TypeFlags.Object | TypeFlags.Intersection);
         }
 
         function instantiateAnonymousType(type: AnonymousType, mapper: TypeMapper): AnonymousType {