Error: List<> does not contain a definition for 'Sum'

Error 'System.Collections.Generic.List<>' does not contain a definition for 'Sum' and no extension method 'Sum' accepting a first argument of type 'System.Collections.Generic.List<>' could be found (are you missing a using directive or an assembly reference?)

This error cropped up when converting a solution from .NET 2 to .NET 4.  Just add

using System.Linq;