Null Parameter In Uniqueidentifier Field

I have the following database structure:



The table aspnet_users, which we all know



Another table with an uniqueidentifier which references the user iD



I want to set the value of the iD in one of the entries in the second table to null.



I tried this:

string assigned is null, i am passing this as a method parameter

...............

string queryString = "UPDATE  [table2] SET  AssignedUserId=@assigned WHERE ProblemId = @id";

        System.Data.IDbCommand dbCommand = new System.Data.SqlClient.SqlCommand();

...................

System.Data.IDataParameter dbParam_au_pr = new System.Data.SqlClient.SqlParameter();

        dbParam_au_pr.ParameterName = "@assigned";

        if (assigned == null)

        {

            dbParam_au_pr.Value = null;

        }

        else

        {

            dbParam_au_pr.Value = assigned;

        }

        dbParam_au_pr.DbType = System.Data.DbType.String ;

        dbCommand.Parameters.Add(dbParam_au_pr);



I also tried using "" instead of null, or not using that "if" statement at all.

I get an error that says:



Parameterized Query '(@id nvarchar(1),@assigned
nvarchar(4000))UPDATE  [tracker_Probl' expects parameter @assigned,
which was not supplied.
Please help with this as soon as possible.Thanks