diff --git a/packages/zone.js/lib/zone-impl.ts b/packages/zone.js/lib/zone-impl.ts
index d943fcff7c5..9b47fa2b2e7 100644
--- a/packages/zone.js/lib/zone-impl.ts
+++ b/packages/zone.js/lib/zone-impl.ts
@@ -1128,7 +1128,7 @@ export function initZone(): ZoneType {
     }
     private _zone: ZoneImpl;
 
-    private _taskCounts: {microTask: number; macroTask: number; eventTask: number} = {
+    private _taskCounts: {[key in TaskType]: number} = {
       'microTask': 0,
       'macroTask': 0,
       'eventTask': 0,