},
	}

	for _, testCase := range testCases {
		err := testCase.fn()
		if err.Type != testCase.expected {
			t.Errorf("expected Type %q, got %q", testCase.expected, err.Type)
		}
	}
}

func TestErrorUsefulMessage(t *testing.T) {
	{
		s := Invalid(nil, nil, "").Error()
		t.Logf("message: %v", s)
		if !strings.Contains(s, "null") {
			t.Errorf("error message did not contain 'null': %s", s)
		}
	}