Where a mistake in the controller? - asp.net-mvc

Question

Here we have a controller:
public class CollegeController : Controller
{
private ICollegeRepository repository;
public int pageSize = 10;
public CollegeController(ICollegeRepository repo)
{
repository = repo;
}
public ViewResult List(string region,
string area,
string localityType, int page = 1)
{
CollegeListViewModel model = new CollegeListViewModel
{
Colleges = repository.Colleges
.Where(p => (region == null || p.Area.Region.Name == region)
&& (localityType == null || p.LocalityType.Name == localityType))
.Where(p => area == null || p.Area.Name == area)
.OrderBy(college => college.CollegeId)
.Skip((page - 1) * pageSize)
.Take(pageSize),
PagingInfo = new PagingInfo
{
CurrentPage = page,
ItemsPerPage = pageSize,
TotalItems = ((region == null) && (localityType == null)) ?
repository.Colleges.Count() :
repository.Colleges.Where(college => college.Area.Region.Name == region)
.Where(college => college.Area.Name == area)
.Where(college => college.LocalityType.Name == localityType).Count()
},
CurrentLocalityType = localityType,
CurrentRegion = region,
CurrentArea = area,
};
return View(model);
}
}
The whole problem is that when you start if you pass the first region, and then localityType, everything is fine.
But if you try to pass localityType, when in the region passed null, localityType also sends null.
In other words, before sending localityType must pass region
How such to correct?
And Sorry for my English