How To Pass Null Value To The Database If A Form Field Is Empty?

How can I pass into the database (@User_fax = null) if the fax form field is empty, from a command type Stored Procedure? For example:





Dim CmdUpdate As New SqlCommand("Form2_NewUser", strConnection)


CmdUpdate.CommandType = CommandType.StoredProcedure





CmdUpdate.Parameters.Add("@User_fax", SqlDbType.char, 9)


CmdUpdate.Parameters("@User_fax").Value = fax.Text()





...





strConnection.open()


CmdUpdate.ExecuteNonQuery


strConnection.close()











And, the stored procedure inside Sql server:





USE market1


GO


ALTER PROC Form2_NewUser


@User_id bigint, @User_fax char(9),...





AS





SET NOCOUNT ON


UPDATE Users


SET User_fax = @User_fax, ...





WHERE User_id = @User_id


SET NOCOUNT OFF


GO











Thank you,


Cesar