I Am Getting An Error While Making A Regular Database Command

hello

here is my code  int a = Convert.ToInt32(Request.QueryString["ProductID"]);
SqlConnection conn = new SqlConnection(ConnectionString);

SqlCommand command4 = new SqlCommand("myStoredProcedure", conn);
command4.CommandType = CommandType.StoredProcedure;

DbParameter param2 = command4.CreateParameter();
param2.ParameterName = "@ProductID";
param2.DbType = DbType.Int32;
param2.Value = a;
command4.Parameters.Add(param2);

param2 = command4.CreateParameter();
param2.Direction = ParameterDirection.Output;
param2.ParameterName = "@UserName";
param2.DbType = DbType.String;
command4.Parameters.Add(param2);

conn.Open();
command4.ExecuteReader();
conn.Close();

Label3.Text = command4.Parameters["@UserName"].Value.ToString();
}Exception Details: System.InvalidOperationException: String[1]: the Size property has an invalid size of 0.
Line 48:
Line 49: conn.Open();
Line 50: command4.ExecuteReader();
Line 51: conn.Close();

 and here is my stored procedure ALTER PROCEDURE dbo.myStoredProcedure
--

(@ProductID int,
@UserName varchar(255) OUTPUT)

AS

SET @UserName = (SELECT UserId FROM Products WHERE ProductID = @ProductID)

 what is wrong? i never seen that error and i don't know from where could it came from

please help me,

thanks