Wouldn't a generic 400 be better. It's not that the page wasn't found, but you've sent something that was not an accepted request. Fix your request and try again is how I've read it, and that's how I use it in the APIs I provide. I prefer it over 406 since it's not my end that can't process it. If your query string is tacking extra stuff trying to break things or just because your request wasn't crafted per the docs, then it's on you.