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?