Required Object has not been asserted

Hi,

My development evironment is NetWeaver 7.3 EhP1 SP05. I created a flow ruleset in rules composer, in one of its gateways, there are two conditions:

1. Position.getEmployee(ID) Not Equals null

2. Position.getEmployee(ID) Equals null

The method Position.getEmployee(ID) returns an object of type Employee. In order to test this flow ruleset, I created a web service with this flow ruleset. When I tested the web service in WS Navigator, an error ocurred:

Web service returned error. Fault Code: "(http://schemas.xmlsoap.org/soap/envelope/)Client" Fault String: "Required Object has not been asserted. Cannot resolve: com.Position(method: getEmployee)".

It seems that I should assert the object Position. But I don't know how to assert it. How should I solve this error?

Best Regards,

Abe