oracle function always return null

I'm using the flowing code to call the oracle function with the return value, but it returns null always

    OracleCommand cmd = new OracleCommand();
    using (OracleConnection cnn = new OracleConnection("Data Source=(DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=321352427544)(PORT=1521))(CONNECT_DATA=(SERVICE_NAME=test)));User ID=abc;Password=123;"))
    {

        cmd.Connection = cnn;
        cmd.CommandText = "GetEmp";
        cmd.CommandType = CommandType.StoredProcedure;

        cmd.Parameters.Add("P_EMP_ID", OracleDbType.Int32).Value = 4241;
    

        cmd.Parameters.Add(new OracleParameter("return_value", OracleDbType.Int32)).Direction = ParameterDirection.ReturnValue;

        cnn.Open();
        cmd.ExecuteNonQuery();

        string Count = (string)cmd.Parameters["return_value"].Value;

        cnn.Close();
    }