Testing a method in a Vue3 setup component with vitest

I need to make a test for a method in a VUE3 setup in a component and nothing I have tried works.

Here is the simplified component:

<script setup>

function myFunction(number) {
   return number + 1
}

</script>

<template>
// code here

And the test:

import { MyComponent } from '@/Components/MyComponent.vue'
import { describe, expect, test } from 'vitest'
import { mount } from '@vue/test-utils'

describe('MyComponent.vue', () => {

    test('myMethodWorks', () => {
        const wrapper = mount(MyComponent)
        const result = wrapper.vm.$setupState.myFunction(1);
        expect(result).toBe(2)
        
    });
});

But I keep getting the error: ReferenceError: document is not defined. There are other tests in the Vue test folder that work fine, so I shouldn’t need to change any config files. But none of the other tests are trying to test a method from a Vue3 component.
Even if I put in the test :
expect(true).toBe(true)
I still get the same error.

What am I doing wrong?