optional groups in XML

I want to call an external web service that receives a nested XML schema. The schema contains many optional groups, i.e. complex types with minoccurs = "0". These in turn contain fields with no minoccurs declaration. I assume this means the fields become mandatory when the group is present.

When I create the function call inside a query it makes all the groups mandatory (presumably because there is a "mandatory" field inside). The result is I am forced to pass unwanted parameters to the web services.

Is this a limitation of Data Services?

Thanks

Cliff