Unit Testing Jest React Mutation

I am mocking my mutation with this. But it gives me TypeError: startSession.mutate is not a function

      <div className="menu">
        <Button
          onClick={() => startSession.mutate(clientId)}>
          Start Session
        </Button>
      </div>

describe('<ChatSessionTimer />', () => {
  const startSession = jest.fn()

    it('calls startSession on button click', async () => {
      const wrapper = mount(<ChatSession customerId="12234" />)
      wrapper.find('Button').find('a.button').at(0).simulate('click')
      expect(startSession).toHaveBeenCalled()
    })
   })

How can I make this work? Thanks a lot.