@Test
public void testSigninShouldFailWithNullUsername() {

    doThrow(new IllegalArgumentException("username == null"))
            .when(api).signin((String) isNull(), any(String.class), any(LoginCallback.class));
    TestObserver<Token> testObserver = sut.signin(null, "password").test();
    testObserver.assertError(IllegalArgumentException.class);
}