Input string was not in a correct format.

public static bool CheckLogin(string username, string password, out int registerId, out int role, out string firstname)
  {
      bool isValid = false;

      registerId = default(int);
      role = default(int);
      firstname = string.Empty;

      using(MySqlConnection conn = new MySqlConnection(Constants.CONNECTION))
      {
          using(MySqlCommand cmd = new MySqlCommand(Constants.Procs.CHECK_LOGIN,conn))
          {
              cmd.CommandType = CommandType.StoredProcedure;

              cmd.Parameters.AddWithValue("_Email", MySqlDbType.VarChar).Value = username;
              cmd.Parameters.AddWithValue("_Password", MySqlDbType.VarChar).Value = password;

              conn.Open();

              using(MySqlDataReader reader = cmd.ExecuteReader())// here am getting error
              {
                  while(reader.Read())
                  {
                      isValid = true;

                      registerId = reader["RegisterID"] == System.DBNull.Value ? 0 : Convert.ToInt32(reader["RegisterID"]);
                      //role = reader["role"] == System.DBNull.Value ? 0 : Convert.ToInt32(reader["role"]);
                      //firstname = Convert.ToString(reader["Firstname"]);
                  }
              }
          }
      }
      return isValid;
  }