Search function oddity - any idea how to fix this?

We use RoboHelp in our call center for our procedure binder - works great, but we've found an odd search function problem. We have several different processes on how to handle our calls when a customer record can't be found and it's called "member not found". But when we try to search for the words "member not found" it returns a message saying "The words you typed is not a valid expression." It's causing us some headaches as it's a common search. Doesn't seem to happen for any other item we've searched for. Entering "not found" works, but not "member not found". Any idea why and how to fix this? Is this combination of words some odd programming language thing?