Vue.js Composition API Watch 配列の指定について

わからない点

watchで配列データを監視対象とする方法を教えてください。
従来では配列データでsample.aを指定する場合、
クォーテーションで囲めば扱えましたが、
Composition APIだとエラーとなります。

###　ソース

js
1<script lang="ts">
2import Vue from "vue";
3import { defineComponent, reactive, watch } from "@vue/composition-api";
4
5export default defineComponent({
6  name: "sampleVue",
7  setup() {
8    const sample = reactive({
9      a: "a"
10    })
11    watch(sample.a,() => {
12      alert("watched")
13    })
14  },
15});
16</script>

エラー

No overload matches this call.
  Overload 1 of 3, '(sources: readonly WatchSource<unknown>[], cb: WatchCallback<readonly unknown[], readonly unknown[]>, options?: WatchOptions<false>): WatchStopHandle', gave the following error.
    Argument of type '"sample.a"' is not assignable to parameter of type 'readonly WatchSource<unknown>[]'.
  Overload 2 of 3, '(source: WatchSource<unknown>, cb: WatchCallback<unknown, unknown>, options?: WatchOptions<false>): WatchStopHandle', gave the following error.
    Argument of type '"sample.a"' is not assignable to parameter of type 'WatchSource<unknown>'.
  Overload 3 of 3, '(source: object, cb: WatchCallback<object, object>, options?: WatchOptions<false>): WatchStopHandle', gave the following error.
    Argument of type '"sample.a"' is not assignable to parameter of type 'object'.Vetur(2769)