An unhandled exception of type 'system.invalidoperationexception' occurred in system.xml.dll

I receive no errors when i compile the program, however as soon as I click the save button I receive 'InvalidOperationException was unhandled'. 
              XmlSerializer serializer = new XmlSerializer(obj.GetTYpe()); Is then highlighted,

<pre lang="c#">
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
using System.Xml.Serialization;

namespace Config_Tool
{
      [Serializable] 
      public saveXML()
        {}
      public static void savedata(object obj, string filename)
      {
              XmlSerializer serializer = new XmlSerializer(obj.GetTYpe());
              using(StreamWriter writer = new Streamwriter(filename)
               {
                     serializer.Serialize(writer. obj);
                }
        }
}