fix(toRef): issue vuejs#855

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> {
@@ -157,6 +158,8 @@ export function toRef<T extends object, K extends keyof T>(
   const v = object[key]
   if (isRef<T[K]>(v)) return v
 
+  if (!(key in object)) set(object, key, undefined)
+
   return createRef({
     get: () => object[key],
     set: (v) => (object[key] = v),