linq query where int ID belongs to List<int>

I'm having a problem with a linq query. I have used this similarly before but i cannot understand what could be wrong now.

Errors

The best overloaded method match for 'System.Collections.Generic.List.Contains(int)' has some invalid arguments
Argument '1': cannot convert from 'int?' to 'int' ; refers to the where clause of rTestResults

Code:

List<int> rMTCPlates = (from rP in mDataContext.Plates
                        where rP.SOItem.SONumberID == aSONumber
                        select rP.ID).ToList();

var rTestResults = from rT in mDataContext.TestSamplesViews
                   where rMTCPlates.Contains(rT.PlateID)
                   select rT;  

Any idea whats going on?

Any help appreciated,
Thanks