This is one of those errors in Rails that just make you go WTF!? The amount of debug information is minimal, and the row that supposedly is the culprit is syntactically correct. Even Google doesn’t really help. I gather that this error can occur in several different contexts and caused by different things, so I’m not trying to provide an all purpose solution to this. The case I’m going to show you is probably quite rare, but it happened to us at work today. I never figured out exactly why Rails chokes in this situation, but I found a way to reproduce the problem and also a solution. The main characters in this drama are the respond_to and render methods, and the main villain is a mysterious invisible parameter.