为什么我不能设置CancellationToken？

Question

我正在尝试为类编写测试，似乎我无法设置我的 CheckLoginAsync 方法。此 CheckLoginAsync 方法有2个参数。第二个参数是可选的，如下所示：

    public async Task CheckLoginAsync(ISettings userSettings, CancellationToken? cancellationToken = null);

设置代码如下：

        backendFacadeMock = new Mock<BackendFacade>(MockBehavior.Strict);
        backendFacadeMock
            .Setup(bf => bf.Initialize())
            .Returns(() => Task.FromResult(0));
        backendFacadeMock
            .Setup(bf => bf.CheckLoginAsync(It.IsAny<ISettings>(), It.IsAny<CancellationToken?>()))
            .Returns(() => Task.FromResult(0));

        nullBackendFacadeMock = new Mock<IBackendFacade>(MockBehavior.Strict);


        backendFacadeFactoryMock = new Mock<IBackendFacadeFactory>(MockBehavior.Strict);
        backendFacadeFactoryMock
            .Setup(bf => bf.CreateBackendFacade())
            .Returns(() => backendFacadeMock.Object);
        backendFacadeFactoryMock
            .Setup(bf => bf.CreateNullBackendFacade())
            .Returns(() => nullBackendFacadeMock.Object);

正在测试的代码如下：

    private async Task<IBackendFacade> CreateBackendFacade()
    {
        IBackendFacade backendFacade = _backendFacadeFactory.CreateBackendFacade();
        try
        {              
            await backendFacade.CheckLoginAsync(_userSettings);            
            await backendFacade.Initialize();
        }
        catch (Exception e)
        {
            backendFacade = _backendFacadeFactory.CreateNullBackendFacade();
        }

        return backendFacade;
    }

上述代码中的 CheckLoginAsync 调用会引发以下异常：

IBackendFacade.CheckLoginAsync(Mock<de.cas.officeintegration.outlook.core.application.ISettings:00000001>.Object, null) invocation failed with mock behavior Strict.
All invocations on the mock must have a corresponding setup.

测试本身：

    [Test]
    public async Task GetBackendFacadeAsync_NoException_ReturnsABackendFacadeObject()
    {
        var backendFacadeProvider = new BackendFacadeProvider(backendFacadeFactoryMock.Object, userSettingsMock.Object);

        IBackendFacade backendFacade = await backendFacadeProvider.GetBackendFacadeAsync();

        Assert.AreSame(backendFacadeMock.Object, backendFacade);
    }

提前致谢！