C# Trying to serialize an ArrayList into an XML file

struct ClientInfo
    {
        public string strName;  //Name by which the user logged into the chat room
        public string strPW;
    }

    ArrayList clientList = new ArrayList();

    public static void Serialize(ArrayList input)
    {
        XmlSerializer serializer = new XmlSerializer(input.GetType());
        TextWriter sw = new StreamWriter("users.txt");
        serializer.Serialize(sw, input);
        sw.Close();
    }

So I am trying to store Name/Password combinations in an ArrayList and I am trying to save this ArrayList into a file, and load it every time the program is started. However, the program stops at the serializer.Serialize(sw, input); line with the following:

An unhandled exception of type 'System.InvalidOperationException' occurred in System.Xml.dll

What am I doing wrong ?