不能在LINQ查询中使用三元运算符

Object reference not set to an instance of an object.

var courses = from d in somesource
                          orderby d.SourceName, d.SourceType
                          select new
                          {
                              ID = d.InternalCode,
                              Name = string.Format("{0} - {1}{2}", d.InternalCode, d.SourceName, (d.SourceType.Length > 0 ? ", " + d.SourceType : string.Empty))
                          };

有什么想法吗?