diff --git a/packages/reactivity/__tests__/reactive.spec.ts b/packages/reactivity/__tests__/reactive.spec.ts
index b7f9cdf4eb3..ab798ff2c8b 100644
--- a/packages/reactivity/__tests__/reactive.spec.ts
+++ b/packages/reactivity/__tests__/reactive.spec.ts
@@ -44,6 +44,24 @@ describe('reactivity/reactive', () => {
     expect(isReactive(observed.array[0])).toBe(true)
   })
 
+  test('process subtypes of collections properly', () => {
+    class CustomMap extends Map {
+      count = 0
+
+      set(key: any, value: any): this {
+        super.set(key, value)
+        this.count++
+        return this
+      }
+    }
+
+    const testMap = new CustomMap()
+    const observed = reactive(testMap)
+    expect(observed.count).toBe(0)
+    observed.set('test', 'value')
+    expect(observed.count).toBe(1)
+  })
+
   test('observed value should proxy mutations to original (Object)', () => {
     const original: any = { foo: 1 }
     const observed = reactive(original)
diff --git a/packages/reactivity/src/reactive.ts b/packages/reactivity/src/reactive.ts
index 4dea9e00c86..e7db1e0a071 100644
--- a/packages/reactivity/src/reactive.ts
+++ b/packages/reactivity/src/reactive.ts
@@ -18,7 +18,9 @@ export const enum ReactiveFlags {
   IS_READONLY = '__v_isReadonly',
   RAW = '__v_raw',
   REACTIVE = '__v_reactive',
-  READONLY = '__v_readonly'
+  READONLY = '__v_readonly',
+
+  IS_COLLECTION = '__v_isCollection'
 }
 
 interface Target {
@@ -28,9 +30,15 @@ interface Target {
   [ReactiveFlags.RAW]?: any
   [ReactiveFlags.REACTIVE]?: any
   [ReactiveFlags.READONLY]?: any
+
+  [ReactiveFlags.IS_COLLECTION]?: boolean
 }
 
-const collectionTypes = new Set<Function>([Set, Map, WeakMap, WeakSet])
+;([Set, Map, WeakMap, WeakSet] as Array<Function>).forEach(
+  collectionType =>
+    (collectionType.prototype[ReactiveFlags.IS_COLLECTION] = true)
+)
+
 const isObservableType = /*#__PURE__*/ makeMap(
   'Object,Array,Map,Set,WeakMap,WeakSet'
 )
@@ -151,9 +159,10 @@ function createReactiveObject(
   if (!canObserve(target)) {
     return target
   }
+
   const observed = new Proxy(
     target,
-    collectionTypes.has(target.constructor) ? collectionHandlers : baseHandlers
+    target[ReactiveFlags.IS_COLLECTION] ? collectionHandlers : baseHandlers
   )
   def(target, reactiveFlag, observed)
   return observed