ErrNoResponseClass Explanation?

I'm pretty new to Ensemble and Cache, and I ran into an error today that I haven't seen before: "ErrNoResponseClass: No Response Classname is assigned for Request class <<someNumber>>". I haven't found much documentation about this beyond the Ensemble Error Codes document, which is essentially just a duplicate of the information I can see in the error message when this problem occurs. Is there any further information about this error and the conditions that cause it? Or perhaps what the <<someNumber>> value represents?

From what I can tell, the response class is set by the base class of my request class, and I can't find anywhere (so far) where this value is set to anything other than its original value. However, what's puzzling me is that this behavior is inconsistent--I can repeat the same operation several times, and sometimes it will succeed, but other times will generate this error message. 

Has anyone run into this sort of behavior before?