I got the same message, but this time the field is a char. (Reverse mapped class, char(1) in database, Char in .NET). Using release 2009.3.1119.2. Is this a known problem?
Dim acls = From a As ACL In scope.Extent(Of ACL)() Where a.[Group].Users.Contains(aUser) and a.allowOrDeny = "A"c
(The class is called ACL, and the char field is called allowOrDeny. VB.NET shown.)