The introduction of self-describing web services has opened up new avenues for the creation of information gathering agents, which are capable of discovering and employing such services at run time to answer user queries. It is desirable for such agents to not only build and execute a query plan, but also specify what information is not returned. In this paper we present a model for expressing the semantics of web services to provide information for such incompleteness analysis. The model relies on an external type system, which, in addition to types, specifies operations that can be performed on the types and properties of these operations. We also describe an algorithm for answering user queries in this model.