Exception: The mapping of interface member IMetaEntity.Url is not supported

Though "Url" is an "LinqToSql" generated property and strictly map to a database field  the exeption still yields when where clause is written as

from item in db.GetTable<T>

where item.Url = "http://xxxxxx"

select item;

Solution:  such writting should be modified as :  item.Url.Equals( "http://xxxxxx" )

Then the exception is avoided.

Why ?  I do not know. Someone points out that it is Linq Team's mistake ...