I have resolved the issue by recreating the depot app from the beginning. I had been avoiding doing so as I was following along with each beta release of the book, and had been building upon the chapters that I had already completed.
I have a feeling that it might have simply been an issue with a bad record in the database, as I recall pressing the “new product” button before completing the code that went along with the button. Unfortunately, I had dropped and recreated the database before I thought to take a look at it.
Please consider this issue closed. Thanks!