XmlSerialization of Custom Type

I've been trying to get this to work all morning and can't seem to get it to work.  Microsoft say's it's not possible to use the XmlSerializer to serialize an array of List<T> and ArrayList according to http://msdn.microsoft.com/en-us/library/system.xml.serialization.xmlserializer.aspx.

However, according to this article and a lot of user comments below it, it says you can.
http://www.hanselman.com/blog/XmlSerializingAGenericListltgt.aspx

This is the error I'm receiving:
There was an error reflecting type 'EFC.SupportServices.Management.Classes.Department'.

This is the InnerException:
"Cannot serialize member 'EFC.SupportServices.Management.Classes.Department.Groups' of type 'System.Collections.Generic.List`1[[EFC.SupportServices.Management.Classes.Group, EFC.SupportServices.Management, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]]', see inner exception for more details."

I don't necessarily have to use List<T>, but need some way of holding a collection of the other types.  So anyway to get this to work will probably suffice.

Here are the classes I'm trying to serialize:

[DataContract(), XmlRoot()]
    [KnownType(typeof(Group))]
    [KnownType(typeof(Role))]
    public class Department
    {
        private List<Group> _Groups;
        private List<Role> _Role;
 
        public string Name { get; set; }
 
        [DataMember()]
        public List<Group> Groups
        {
            get
            {
                if (_Groups == null)
                {
                    _Groups = new List<Group>();
                }
                return _Groups;
            }
            set
            {
                _Groups = value;
            }
        }
 
        [DataMember]
        public List<Role> Role
        {
            get
            {
                if (_Role == null)
                {
                    _Role = new List<Role>();
                }
                return _Role;
            }
            set
            {
                _Role = value;
            }
        }
    }
 
    [DataContract()]
    public class Role
    {
        public Role(string Name)
        {
            Title = Name;
        }
 
        [DataMember()]
        public string Title { get; set; }
 
    }
 
    [DataContract()]
    public class Group
    {
        public Group (string GroupName)
        {
            Name = GroupName;
        }
 
        public Group (string GroupName, string GroupDisplayName)
        {
            Name = GroupName;
            DisplayName = GroupDisplayName;
        }
 
        [DataMember()]
        public string Name { get; set; }
 
        [DataMember()]
        public string DisplayName { get; set; }
    }

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