diff --git a/packages/zone.js/lib/zone-spec/wtf.ts b/packages/zone.js/lib/zone-spec/wtf.ts
index 7da91d744d..c0ff9b160f 100644
--- a/packages/zone.js/lib/zone-spec/wtf.ts
+++ b/packages/zone.js/lib/zone-spec/wtf.ts
@@ -131,7 +131,8 @@
     const out: {[k: string]: any} = {};
     for (const key in obj) {
       if (obj.hasOwnProperty(key)) {
-        let value = obj[key];
+        // explicit : any due to https://github.com/microsoft/TypeScript/issues/33191
+        let value: any = obj[key];
         switch (typeof value) {
           case 'object':
             const name = value && value.constructor && (<any>value.constructor).name;