Unclear error when mocking Enum.GetNames

    public enum Country
    {
        Antartica
    }

        [Test, Isolated]
        public void BigCountry()
        {

            Isolate.WhenCalled(()=>Enum.GetNames(typeof(Country))).WillReturn(new[]{"A"});
        }

TestCase 'BigCountry' failed: System.NullReferenceException : Object reference not set to an instance of an object.
   at c9.a(et A_0)
   at c9.a(Object A_0)
   C:Documents and SettingsSoon Hui.ESTEEMSOFTMy Documentsjobs	hird party components	ypemockEnumMockEnumMockEnumMockCountryTest.cs(23,0): at EnumMock.CountryTest.BigCountry()
   at TypeMock.MockManager.a(String A_0, String A_1, Object A_2, Object A_3, Boolean A_4, Object[] A_5)
   at TypeMock.InternalMockManager.getReturn(Object that, String typeName, String methodName, Object methodParameters, Boolean isInjected)
   C:Documents and SettingsSoon Hui.ESTEEMSOFTMy Documentsjobs	hird party components	ypemockEnumMockEnumMockEnumMockCountryTest.cs(20,0): at EnumMock.CountryTest.BigCountry()