XmlSerializer exception with ArrayList

[Chuck Bowling]

I'm not sure what I'm doing wrong here. I'm getting the exception

"An unhandled exception of type 'System.InvalidOperationException' occurred
in system.xml.dll

Additional information: There was an error generating the XML document."

This exception only showed up after I added the ArrayList and populated it
with an instance of RepetitionEvent.

I'm trying to serialize to Xml the class below using this function:

//--------------[ frmQuizzer.cs ]----------------------------
private void SerializeQuiz()
{
XmlSerializer serializer = new XmlSerializer(typeof(CodeSampleQuiz));
TextWriter writer = new StreamWriter("test.xml");
serializer.Serialize(writer, t);
writer.Close();
}
//-----------[ CodeSampleQuiz.cs ]-----------------------

namespace SampleQuizzer
{
public class RepetitionEvent
{
public DateTime date;
public TimeSpan duration;
}

public class CodeSampleQuiz
{
private DateTime startTime;
public ArrayList repetitionEvents = new ArrayList();
string question, answer;

public CodeSampleQuiz()
{
}

// stuff...

public void LogAnswer()
{
RepetitionEvent e = new RepetitionEvent();
e.date = DateTime.Now;
e.duration = DateTime.Now - startTime;
repetitionEvents.Add(e);
}
}
}