Passing an instance of anonymous type over WCF

I have a WCF service method that expects an object and then retrieves its properties using reflection.

On the client side I create an anonymous type object

var obj = new {FirstName="John", LastName="Doe"}

and pass it to the method. I'm getting an exception:

Type '<>f__AnonymousType0`2[System.String,System.String]' cannot be serialized. 
Consider marking it with the DataContractAttribute attribute, and marking all 
of its members you want serialized with the DataMemberAttribute attribute.  
See the Microsoft .NET Framework documentation for other supported types.

I can't mark the type or its members with serialization attributes because there is really no type nor properties declared explicitly. Is there a way to overcome that problem?