TypeScript error in my Vue component: TS2339: Property 'X' does not exist on type '{ myFunction(): void; }'

I have created a Vue2 component that I want to place inside another Vue 2 component. I have created the component inside a TypeScript file.

const ChildrenAgeDropdown = {
  props: ["option"],
  data() {
    return {
      isOpen: false as boolean,
      options: arrayFromXToY(0, 11),
    };
  },
  methods: {
    toggle(): void {
      this.isOpen = !this.isOpen;
    },
  },

// ... The rest of the component.
}

The code works perfectly and the program doesn't crash.

However, I get the following Typescript error:

TS2339: Property 'isOpen' does not exist on type '{ toggle(): void; }'

I don't know why I. As you can see, I have declared the property isOpen as a property of the Vue component. What should I do to remove this annoying Typescript Error?

Here is a snapshot of my IDE (WebStorm) with the error: