Type mismatch between client and the service

Hi,

I'm using WCF Data Services 5.1 and have three core assemblies:

1. A Domain assembly that contains POCO classes that are each decorated with a [DataServiceKey] attribute
2. A Host assembly that hosts the data services (with a reference to the domain assembly)
3. A unit test assembly that consumes the hosted data (with a reference to the domain assembly)

The exception I get when I consume the services is:

There is a type mismatch between the client and the service. Type 'XXX' is not an entity type, but the type in the response payload represents an entity type. Please ensure that types defined on the client match the data model of the service, or update the service reference on the client."

The really strange thing is that if I copy the POCO as-is and create it in in the unit test assembly then it works fine.

Can't for the life of my figure it out, any ideas?