ORA-06502 error occurred while calling stored procedure from C#

Here is a Oracle stored procedure.

create or replace procedure GETVPNOFROMPNO(p_pno in varchar2, ret out varchar2) is
      r SYS_REFCURSOR;
      npno1 varchar2(255);
      retval varchar2(255);
begin
 retval:= p_pno;
 open r for
      select npno from parts_for_update_npno where  parts_for_update_npno.pno= p_pno and npno is not null;
 fetch r into npno1;
 if r%NOTFOUND then
    ret:= retval;
    return;   
 end if;
 retval:= p_pno||' /'||npno1;
 ret:= retval;
end GETVPNOFROMPNO;

As you see, there is no need of REF CURSOR. I declared it just for further use.
And here is a function that calls the stored procedure above.

       public static string getVPNOfromPNO(OracleConnection c, string pno)
       {
           OracleCommand cmd = null;
           try
           {
               c.Open();
               cmd = c.CreateCommand();
               cmd.CommandText = "GETVPNOFROMPNO";
               cmd.CommandType = CommandType.StoredProcedure;

               cmd.Parameters.Add("p_pno", OracleType.VarChar);
               cmd.Parameters["p_pno"].Value = pno;
               cmd.Parameters["p_pno"].Direction = ParameterDirection.Input;

               cmd.Parameters.Add("ret", OracleType.VarChar);
               cmd.Parameters["ret"].Value = "";
               cmd.Parameters["ret"].Direction = ParameterDirection.Output;

               cmd.ExecuteNonQuery();
               c.Close();
               //MessageBox.Show("Suces");
           }
           catch (Exception ex)
           {
               MessageBox.Show(ex.ToString());
               Program.setStatus("Error while getting ...!", ex);
           }
           return cmd.Parameters["ret"].Value.ToString();
       }

Do you think there is any error? I thought not, but there was.
While executing, this code throws "ORA-06502: numeric or value error: character string buffer too small" exception.

The error is corrected by changing cmd.Parameters["ret"].Value = ""; to cmd.Parameters["ret"].Value = "__________________";

Why? I used parameter for getting output value. The too small string buffer in the error description is cmd.Parameters["ret"].Value that is length of zero. After setting enough space for this variable, it was OK. If i used oracle function, it would be OK without any problem.