diff --git a/src/mixin.ts b/src/mixin.ts
index 3dc3b191..9bdc197a 100644
--- a/src/mixin.ts
+++ b/src/mixin.ts
@@ -149,8 +149,8 @@ export function mixin(Vue: VueConstructor) {
     }
   }
 
-  function customReactive(target: object, visited = new Set()) {
-    if (visited.has(target)) return
+  function customReactive(target: object, seen = new Set()) {
+    if (seen.has(target)) return
     if (
       !isPlainObject(target) ||
       isRef(target) ||
@@ -166,8 +166,8 @@ export function mixin(Vue: VueConstructor) {
       const val = target[k]
       defineReactive(target, k, val)
       if (val) {
-        visited.add(val)
-        customReactive(val, visited)
+        seen.add(val)
+        customReactive(val, seen)
       }
       return
     })
diff --git a/src/reactivity/reactive.ts b/src/reactivity/reactive.ts
index 291a990f..86f3e063 100644
--- a/src/reactivity/reactive.ts
+++ b/src/reactivity/reactive.ts
@@ -135,11 +135,11 @@ export function observe<T>(obj: T): T {
 /**
  * Mock __ob__ for object recursively
  */
-function mockReactivityDeep(obj: any, seen = new WeakMap<any, boolean>()) {
+function mockReactivityDeep(obj: any, seen = new Set()) {
   if (seen.has(obj)) return
 
   def(obj, '__ob__', mockObserver(obj))
-  seen.set(obj, true)
+  seen.add(obj)
 
   for (const key of Object.keys(obj)) {
     const value = obj[key]