Error assigning null to a nullable int - "The value 'null' is not valid for property"

I have this property in my view model:

[DisplayName("Region")]
public int? RegionId { get; set; }

I pass my view model to my controller, and it fails at ModelState.IsValid if RegionId is null. If I pass an integer to it, it works fine.

The error message is:

The value 'null' is not valid for Region

I've also tried calling this before I check ModelState.IsValid, but I get the same error:

if (viewModel.RegionId == null)
    viewModel.RegionId = (int?)null;

What's the problem here? Why can't I assign null to something that is nullable?