LUIS, bot framework, and the 'None' intent

So my bot was handling words like 'fsdfd' correctly, but any real words, for which I did not have an utterance defined in any intent, were being handled unexpectedly. For example, 'blue', 'student', and 'dog' were all being handled by the 'Cancel' intent.

The answer was to add just a few of these utterances to the 'None' intent; after that, the engine correctly handled all these words -- and plenty more -- with the 'None' intent. It just needed that bit of training to get started.