Vue+TS/Typescript：Property does not exist on type ‘(() =＞ any) | ComputedOptions＜any＞‘

interface IMenuButton {
  top: number // 上边界坐标，单位：px
  right: number // 右边界坐标，单位：px
  bottom: number // 下边界坐标，单位：px
  left: number // 左边界坐标，单位：px
}
export default Vue.extend({
  props: {
    menuButton: {
      type: Object as PropType<IMenuButton>,
    },
  },
  computed: {
    styleString() {
      if (this.menuButton) {
        return `padding-top: ${this.menuButton.top}px;`
      } else {
        return 'padding-top: 50px;'
      }
    },
  }
})

TS2339: Property 'top' does not exist on type '(() => any) | ComputedOptions<any>'.
  Property 'top' does not exist on type '() => any'.
    41 |     styleString() {
    42 |       if (this.menuButton) {
  > 43 |         return `padding-top: ${this.menuButton.top}px;`
       |                                                ^^^
    44 |       } else {
    45 |         return ''
    46 |       }

  computed: {
    styleString(): string {
      if (this.menuButton) {
        return `padding-top: ${this.menuButton.top}px;`
      } else {
        return 'padding-top: 50px;'
      }
    },
  }