@Test
public void testSigninShouldFailWithNullPassword() {

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