diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 0c91e1ba062..b595d77295a 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -12043,7 +12043,7 @@ namespace ts {
         }
 
         function isStringIndexSignatureOnlyType(type: Type): boolean {
-            return type.flags & TypeFlags.Object && getPropertiesOfType(type).length === 0 && getIndexInfoOfType(type, IndexKind.String) && !getIndexInfoOfType(type, IndexKind.Number) ||
+            return type.flags & TypeFlags.Object && !isGenericMappedType(type) && getPropertiesOfType(type).length === 0 && getIndexInfoOfType(type, IndexKind.String) && !getIndexInfoOfType(type, IndexKind.Number) ||
                 type.flags & TypeFlags.UnionOrIntersection && every((<UnionOrIntersectionType>type).types, isStringIndexSignatureOnlyType) ||
                 false;
         }