diff --git a/packages/compiler/src/output/output_ast.ts b/packages/compiler/src/output/output_ast.ts
index 7b456e89d6..82d5addb91 100644
--- a/packages/compiler/src/output/output_ast.ts
+++ b/packages/compiler/src/output/output_ast.ts
@@ -1464,7 +1464,7 @@ class _ApplySourceSpanTransformer extends AstTransformer {
   constructor(private sourceSpan: ParseSourceSpan) { super(); }
   private _clone(obj: any): any {
     const clone = Object.create(obj.constructor.prototype);
-    for (let prop in obj) {
+    for (let prop of Object.keys(obj)) {
       clone[prop] = obj[prop];
     }
     return clone;