Simple checkbox test with vue unit testing library

Trying to work out something that I thought would be pretty simple!

I have a PrimeVue checkbox component on my page (and I'm using Vue 3, PrimeVue, vitest, @testing-library/vue and @testing-library/user-event)

<div class="field-checkbox">
    <Checkbox role = "checkbox" :aria-label="`Select all students in class ${props.className}`" @change="selectAllChecked()" v-model="selectAll" :binary="true" />
    <label>Select All</label>
</div>

I just want to write a test to check the checkbox and then test that it is correctly checked - not a particularly meaningful test but I'm new to these libraries (and unit testing) so want to work out how to do this stuff.

In my test I have:

it("checkbox test", async() => {
    const ariaLabel = `Select all students in class ${mockProps.className}`;
    const checkbox = screen.getByRole("checkbox", { name: ariaLabel });
    await userEvent.click(checkbox);
    expect(checkbox.getAttribute('checked')).toBe(true)
});

if I remove the 3rd and 4th lines, the test passes so it seems to be checking that the checkbox is there ok.

Lines 3 and 4 (I hoped) would allow me to confirm that the checkbox is now checked but my test fails with

AssertionError: expected null to be true // Object.is equality
  - Expected   "true"
  + Received   "null"

on the 4th line