Web service with optional data inputs

Does anybody know how to make a data input option in a webservice? Below is part of a sample function. As it stands, if one of the inputs is not present in the xml message, an error is returned. I need to make the ANSWER a optional input.

Public Function CreateDoc(ID As String,SYSTEM As String,_

SYSTEM_ID As String, Title As String,QUESTION As String,_

ANSWER As String, returnFault As WS_FAULT) As KAReturnArray