DataContract definition problem

Question

Hi,

I am trying to use the following class in a DataContract:

public class IDParams : Collection<ItemPair<string, List<int>>>
{
}

Do I have to mark it in any way? 

Following is the DataContract I used:
 [DataContract]
 public class Test
 {
     [DataMember]
     public IDParams Item{ get; set; }
 }

It raises a CommunicationException (Not found) :(
Any ideas how I can solve this?

Best regards,
Matthias