WCF Nightmare

Question

Hi,
I am new to WCF and am having some major difficulties.
I am using a hosting application developed by another developer in my organisation.
The problem I am having is when I try to send my populated object across the wire I get "Object reference not set to an instance of an object".
A little about my class structure:
I have a class design for patient data.
It is made up of many Collection properties e.g. IList<PatientAddress> patientAddresses, IList<PatientIdentifier> patientIdentifiers.  If I populate one of the ILists it works fine, but when I attempt to populate more than one I get the above error.
Please Help...
my head is sore from the repeated battering on the wall...