How to detect if a nullable field is missing from biztalk orchestration input?

I have set-up a BizTalk 2010 Orchestration which receives a xml which contains a number of fields (this orch. is exposed as a web service). Some fields are int and nullable so the caller can provide nothing for them. In this case, when my orchestration's "Message Assignment" tries to read those fields and assign their value to another message, everything breaks down and the instance is suspended. (All input fields are distinguished so I can use them in message assignment).

The question is: How can I detect that a field's value is present in the input of orchestration?

(BTW using a default value for the field did not solve the problem)