This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
Since you are adding a client-side layer between client program and web service, you can design it so that a web service call is never made with "incorrect" arguments. You shouldn't change the value in the wrapper layer, but return an error message to the client program.
subject: What should be the behavior of a good wrapper library?