diff --git a/src/reactivity/ref.ts b/src/reactivity/ref.ts
index 87b0ad77..a7a73a44 100644
--- a/src/reactivity/ref.ts
+++ b/src/reactivity/ref.ts
@@ -2,6 +2,7 @@ import { RefKey } from '../utils/symbols'
 import { proxy, isPlainObject, warn, def } from '../utils'
 import { reactive, isReactive, shallowReactive } from './reactive'
 import { readonlySet } from '../utils/sets'
+import { set } from './set'
 
 declare const _refBrand: unique symbol
 export interface Ref<T = any> {
@@ -154,6 +155,7 @@ export function toRef<T extends object, K extends keyof T>(
   object: T,
   key: K
 ): Ref<T[K]> {
+  if (!(key in object)) set(object, key, undefined)
   const v = object[key]
   if (isRef<T[K]>(v)) return v
 
diff --git a/test/apis/computed.spec.js b/test/apis/computed.spec.js
index af0ce83b..da7c8300 100644
--- a/test/apis/computed.spec.js
+++ b/test/apis/computed.spec.js
@@ -1,5 +1,12 @@
 const Vue = require('vue/dist/vue.common.js')
-const { ref, computed, isReadonly, reactive, isRef } = require('../../src')
+const {
+  ref,
+  computed,
+  isReadonly,
+  reactive,
+  isRef,
+  toRef,
+} = require('../../src')
 
 describe('Hooks computed', () => {
   beforeEach(() => {
@@ -195,6 +202,26 @@ describe('Hooks computed', () => {
     expect(app.$children[1].example).toBe('B')
   })
 
+  it('should watch a reactive property created via toRef', (done) => {
+    const spy = jest.fn()
+    const vm = new Vue({
+      setup() {
+        const a = reactive({})
+        const b = toRef(a, 'b')
+
+        return {
+          a,
+          b,
+        }
+      },
+    })
+    vm.$watch('b', spy)
+    vm.b = 2
+    waitForUpdate(() => {
+      expect(spy).toHaveBeenCalledWith(2, undefined)
+    }).then(done)
+  })
+
   it('should be readonly', () => {
     let a = { a: 1 }
     const x = computed(() => a)