Search not working for foreign key fields in MVC

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MVC5

Hi,
I have implemented a search on my MVC Controller. Here I am not able to search on foreign key fields. As it shows Null Reference Exception. Please find the code.

var data = from s in db.User.Include(u => u.Role) select s;
var val = from r in data.AsEnumerable() select r;

if(!String.IsNullOrEmpty(searchString))
{
data = data.Where(s => s.RowNo.ToString().ToUpper().Contains(searchString.ToUpper()) || s.UserName.ToString().ToUpper().Contains(searchString.ToUpper())|| s.RoleID.Role.ToString().ToUpper().Contains(searchstring.ToUpper())).OrderBy(x => x.UserID);
}

So the bold is foreign that I want to have search. But it is showing error like NullReference object.

I am displaying all the data of user table and RoleTable RoleName on my List View. So I have a search box and I am able 2 search on all fields apart from the Foreign key Field.

Please help me in fixing dis.