How to get the sum of list of shorts using the extension method Sum()?

I was trying to do something like this -

List<short> listofshorts= new List<short>();
int s = listofshorts.Sum();
//this does not work...but same code works for a list of ints..

I got this compilation error -

'System.Collections.Generic.List' does not contain a definition for 'Sum' and the best extension method overload 'System.Linq.Queryable.Sum(System.Linq.IQueryable)' has some invalid arguments

Can anyone suggest how can I use an extension method to calculate the sum of shorts? For some reason the extension method does not support it ...