diff --git a/modules/@angular/core/src/util/decorators.ts b/modules/@angular/core/src/util/decorators.ts
index 76899049a5..1c1f5093af 100644
--- a/modules/@angular/core/src/util/decorators.ts
+++ b/modules/@angular/core/src/util/decorators.ts
@@ -26,7 +26,7 @@ export interface ClassDefinition {
    * Other methods on the class. Note that values should have type 'Function' but TS requires
    * all properties to have a narrower type than the index signature.
    */
-  [x: string]: Type | Function | any[];
+  [x: string]: Type | Function | any[] | undefined;
 }
 
 /**