Need help mocking a generic method within a LINQ framew

Hi. I've got a generic method that tests the existence of an entity in a table:

public class Repository
   private readonly MyDataContext db;

   private bool EntityExists<TEntity>(TEntity entity) where TEntity : class
        {
            //return db.GetTable<TEntity>().Any(e => e == entity);
            IQueryable<TEntity> table = db.GetTable<TEntity>();
            bool exists = table.Any<TEntity>(e => e == entity);

            return exists;
        }
}

In my test I mock the data context as follows:

MockManager.Init();
Mock mockDataContext = MockManager.Mock(typeof(TokServicesDataContext));
List<Category> mockCategories = new List<Category>();
mockDataContext.AlwaysReturn("GetTable", mockCategories);
MyRepository rep = new ServicesRepository(new ServicesDataContext(""));

And test like this:

Category cat = new Category{ ID = 1, Name = "A cat", Code = "ABCD1234" };
mockCategories.Add(cat);
bool exists = rep.EntityExists<Category>(cat);
Assert.IsTrue(exists);

When I run the test, it mocks successfully, passing the mock-categories list to DataContext.GetTable<Category>().
However when I run the Any<TEntity>() method it crashes, with a
System.EntryPointNotFoundException. The message was a non descriptive "Entry point was not found".

I can't figure out what is wrong here. Could anybody explain it to me?
Thanks,
Assaf.