diff --git a/src/core/observer/index.js b/src/core/observer/index.js
index 545adb64025..3277246ea2a 100644
--- a/src/core/observer/index.js
+++ b/src/core/observer/index.js
@@ -217,7 +217,7 @@ export function set (target: Array<any> | Object, key: any, val: any): any {
   if (target._isVue || (ob && ob.vmCount)) {
     process.env.NODE_ENV !== 'production' && warn(
       'Avoid adding reactive properties to a Vue instance or its root $data ' +
-      'at runtime - declare it upfront in the data option.'
+      `at runtime - declare it upfront in the data option. Property being added: "${key}"`
     )
     return val
   }