object reference error when define oracle transaction

Question

I am facing this error when define the oracle transaction "Object reference not set to an instance of an object". when trying to define the transaction OracleTransaction transaction = connection.BeginTransaction(IsolationLevel.ReadCommitted)

 public int RunOracleTransaction(Student s, Marks[] m, Course[] c) {
 //TODO: validate s, m, c

using (OracleConnection connection = new OracleConnection(connectionString)) {
connection.Open();

using (OracleCommand command = connection.CreateCommand()) {
  using (OracleTransaction transaction = connection.BeginTransaction(IsolationLevel.ReadCommitted)) {
    command.Transaction = transaction;

    try {
      // Insert the student
      //TODO: put actual query here 
      command.CommandText = 
        @"insert into Students(name)
               values (:prm_Name)
            returning id into :prm_id"; // <- we want inserted student's id

      //TODO: check actual RDBMS types 
      command.Parameters.Add(":prm_Name", OracleType.VarChar).Value = s.Name;
      command.Parameters.Add(":prm_Id", OracleType.VarChar).Direction = ParameterDirection.Output;

      command.ExecuteNonQuery(); 

      string studentId = Convert.ToString(comm.Parameters[":prm_Id"].Value);

      // Insert his/her marks
      command.Parameters.Clear(); // <- forget all prior parameters

      //TODO: put actual query here 
      command.CommandText = 
        @"insert into StudentsMarks(student_Id, mark)
               values (:prm_Student_Id, :prm_Mark)";

      //TODO: check actual RDBMS types 
      command.Parameters.Add(":prm_Student_Id", OracleType.VarChar).Value = studentId;
      command.Parameters.Add(":prm_Mark", OracleType.Int32);

      // insert each mark (in a loop)
      foreach (var mark in m) {
        command.Parameters[":prm_Mark"].Value = m.Mark;
        command.ExecuteNonQuery();  
      }

      // Finally, commit all the inserts
      transaction.Commit();
    }
    catch (DataException e) {
      transaction.Rollback();

      Console.WriteLine(e.ToString());
      Console.WriteLine("Neither record was written to database.");
    }
  }
} 
}

  //TODO: your method returns integer value, please return it  
}