Why doesn't RelayCommand RaiseCanExecuteChanged work in a unit test?

I'm using the current version of MvvmLight available on Nuget (4.1.23.0) and calling RaiseCanExecuteChanged does not appear to be doing anything in a unit test. The scenario is very simple, I have a command:

public RelayCommand FooCommand { get; private set; }

I new it up in the view model constructor and point it to some private methods:

FooCommand = new RelayCommand(Foo, CanFoo);

private void Foo()
{
    // do some fooing.
}

private bool CanFoo()
{
    return SomeRequiredProperty != null;
}

Then in the setter for SomeRequiredProperty I call RaiseCanExecuteChanged:

public object SomeRequiredProperty
{
    get
    {
        return someRequiredProperty;
    }

    set
    {
        someRequiredProperty = value;
        FooCommand.RaiseCanExecuteChanged();
    }
}

Now in a unit test I do the following:

// Arrange
var canExecuteChanged = false;
viewModel.FooCommand.CanExecuteChanged += (sender, args) => canExecuteChanged = true;

// Act
viewModel.SomeRequiredProperty = new object();

// Assert
Assert.That(canExecuteChanged, Is.True);

The test fails because my event handler is not firing. Why is that?

Update: The behaviour does indeed work at run time.