(DataTypes.LookupId) "mystring" result in null?

Feb 3 at 9:52 AM
hi all,

i'm using the following syntax for my query building:
  SPUser currentUserLocal;

            var expressions = new List<Expression<Func<SPListItem, bool>>>();

            foreach (string loginName in ...))
                  string userId = loginName;

                expressions.Add(x => (string)x[SPFieldNames.CaseResponsible.ToString()] == ((DataTypes.LookupId)userId).ToString());
this results in a null pointer exception once I try to generate the query.
Also I notice in the debugger that the expression
results in null.

If I look in the source code for the class "BaseFieldType" I see null is returned?
And the class 'LookupId' has a todo in it. Am I using the wrong version or something?
As the main page states you can use LookupId in the query builder (quick start section)
public class BaseFieldType
        public static explicit operator BaseFieldType(string s) { return null; }

 public class LookupId : BaseFieldTypeWithOperators
            // todo: allow cast from integer to LookupId
//            public static explicit operator LookupId(int id)
//            {
//                return null;
//            }