Error Inserting UserId In Codebehind Using SqlDataSource.insert()

I have a database with columns
userOwnLists
userID uniqueidentifier
userName nvarchar100
userList nvrachar100
createdDate

I
have created successfully a gridview controller to edit these values in
database. The Gridview data is populated by SqlDataSource.

I
have also created a EmptyDataTemplate  and created a form into it.
There is only one textBox and submit button to create the First entry
to userOwnLists -table.

Now I collect the value from EmptyDataTemplate textbox with id userList1 and create a codebehind logic for the submitbutton.


protected void Button2_Click(object sender, EventArgs e)
    {
        TextBox listName = (TextBox)this.FindControl("listName1", GridView1.Controls);
SqlDataSource1.InsertParameters["userId"].DefaultValue = Membership.GetUser().ProviderUserKey;
        SqlDataSource1.InsertParameters["userName"].DefaultValue = Membership.GetUser().UserName.ToString();
        SqlDataSource1.InsertParameters["listName"].DefaultValue = listName.Text;
        SqlDataSource1.InsertParameters["createdDate"].DefaultValue = DateTime.Now.ToString();
        SqlDataSource1.Insert();
    }


The problem is now that I get error:
Exception Details: System.Data.SqlClient.SqlException:
Implicit conversion from data type sql_variant to uniqueidentifier is
not allowed. Use the CONVERT function to run this query.

OK.  So I Googled a bit and found this:
http://scottonwriting.net/sowblog/posts/4690.aspx

My Question is: How do I convert userId so I can insert it to database successfully?

This does not work:

String userId = Membership.GetUser().ProviderUserKey.ToString();
        SqlDataSource1.InsertParameters["userId"].DefaultValue = Convert.ToString(userId);