XmlSerializer.Serialize exception: "The type [type] was not expected."

I'm trying to serialize this class:

[XmlRoot("ArrayOfEvent")]
public class EventList
{
    public EventList()
    {

    }

    public EventList(IEnumerable<Event> items)
    {
        Items = items;
    }

    [XmlArray("")]
    public IEnumerable<Event> Items { get; set; }
}

Here's the Event class contained in EventList.Items:

public class Event
{
    [XmlElement]
    public string ID { get; set; }

    [XmlElement]
    public string Title { get; set; }

    [XmlElement]
    public string Description { get; set; }

    [XmlElement]
    public string Location { get; set; }

    [XmlElement]
    public DateTime? StartTime { get; set; }

    [XmlElement]
    public DateTime? EndTime { get; set; }

    [XmlElement]
    public bool? AllDay { get; set; }
}

And here's where the error occurs:

public static XmlDocument ToXmlDocument<T>(T obj)
{
    var xmlDocument = new XmlDocument();
    var nav = xmlDocument.CreateNavigator();

    if (nav != null)
    {
        using (var writer = nav.AppendChild())
        {
            var ser = new XmlSerializer(typeof(T));
            ser.Serialize(writer, obj); //throws exception
        }
    }

    return xmlDocument;
}

The XmlSerializer gives me this error when calling Serialize():

The type Domain.EventList was not expected. 
Use the XmlInclude or SoapInclude attribute to specify types that are not 
known statically.

I've tried using [XmlInclude(typeof(EventList))] on the EventList class as well as the Event class but neither of those ideas worked. I'm not sure what else to do.

Is anybody familiar with this issue? How can I resolve it? Thanks in advance.