test mapDispatchToProps async actions

I am trying to test my mapDispatchToProps function when an asyncronous function is dispatched. I have read Dan Abramov's suggestions on how to test mapDispatchToProps and I am trying to test my code as such.

I am getting the error...

 TypeError: Cannot read property 'then' of undefined

Here is my test...

describe("mapDispatchToProps", () => {

    const dispatchSpy = jest.fn();
    const {signupUser} = mapDispatchToProps(dispatchSpy);

    it("should dispatch signupActions.signupUser()", () => {
        // mockAxios.onPost(endpoints.SIGNUP,{})
        //         .reply(200,'test_success');
        // tried with and without mockAxios

        signupUser({})
        const spyLastCall = dispatchSpy.mock.calls[0][0];
        expect(spyLastCall)
            .toEqual(signupActions.signupUser({}));
    })

})

The function that I want to test...

export const mapDispatchToProps = dispatch => {
    return { signupUser: (user) => {
        dispatch(signupActions.signupUser(user))
            .then((response) => {
                // do something on success
            }, (error) => {
                // do something on failure
            })
    }
}

I have already tested signupActions.signupUser and I know that it returns a promise. Here is the code...

export function signupUser(user) {
    return (dispatch) => {
        return dispatch(rest.post(SIGNUP,user))
            .then((response) => {
                return Promise.resolve(response);
            },(error) => {
                return Promise.reject(error)
            }
        )
}}

What am I doing wrong?

Ps: I also tried:

 const dispatchSpy = jest.fn().mockImplementation( () => {
    return p = new Promise((reject,resolve) => {
        resolve({})
    }) 
 }

with the same result