Isolate.Verify does not check property set

I cannot get the below test to pass:

        
        [Isolated]
        [TestMethod]
        public void UseSymCryptographyToSetAKey()
        {
            _symCrypt = new SymCryptography();
            Isolate.WhenCalled(() => _symCrypt.Encrypt("Test")).WillReturn("jljljl");
            Isolate.WhenCalled(() => _symCrypt.Key = "key").CallOriginal();
            Isolate.WhenCalled(() => _symCrypt.Key).CallOriginal();
            _adapter = new CryptographyAdapter(_symCrypt);

            _adapter.SetKey("key");
            var encrypted = _adapter.Encrypt("Test");

            try
            {
                Assert.IsFalse(encrypted == "Test");
                Isolate.Verify.WasCalledWithAnyArguments(() => _symCrypt.Key);
            }
            catch (System.Exception e)
            {
                Assert.Fail(e.ToString());
            }
        }

In the method call _adapter.Encrypt I have the following code:

        public override string Encrypt(string value)
        {
            _symCrypt.Key = Key;
            return _symCrypt.Encrypt(value);
        }

Whatever I do, (also tried mocking up the SymCryptography) I get the error:

Microsoft.VisualStudio.TestTools.UnitTesting.AssertFailedException was unhandled by user code
Message=Assert.Fail failed. TypeMock.VerifyException:
TypeMock Verification: Method Informedica.SecureSettings.Cryptographers.SymCryptography.Key was expected but was not called
at em.a(ji A_0, d2 A_1, ho A_2, Predicate`1 A_3)
at b7.b(Delegate A_0)
at b7.WasCalledWithAnyArguments[T](Func`1 func)
at Informedica.SecureSettings.Tests.CryptographyAdapterShould.UseSymCryptographyToSetAKey() in C:DevelopmentSecureSettingsInformedica.SecureSettings.TestsCryptographyAdapterShould.cs:line 29
Source=Microsoft.VisualStudio.QualityTools.UnitTestFramework
StackTrace:
at Microsoft.VisualStudio.TestTools.UnitTesting.Assert.HandleFail(String assertionName, String message, Object[] parameters)
at Microsoft.VisualStudio.TestTools.UnitTesting.Assert.Fail(String message, Object[] parameters)
at Microsoft.VisualStudio.TestTools.UnitTesting.Assert.Fail(String message)
at Informedica.SecureSettings.Tests.CryptographyAdapterShould.UseSymCryptographyToSetAKey() in C:DevelopmentSecureSettingsInformedica.SecureSettings.TestsCryptographyAdapterShould.cs:line 33
InnerException:

But I do call SymCryptography.Key, I checked that in the debugger. However when I add Key = _symCrypt.Key to th Encrypt method, the test passes!!??

So, why can I not just check the property set method?