Error on trying to create node: "could not find corresponding mapping entry"

I've now run into an issue that is (as far as I can tell) unrelated to the issue I posted about earlier tonight. After tweaking the operator_isbndb.inc to include a few more fields, running several test searches to make sure it was working, and setting up a content type in which all the fields corresponded to the results I was getting, I tried actually creating a node, and got the following error message: "could not find corresponding mapping entry"

But all my fieldnames correspond exactly to the attribute names in the ISBNdb response! I even went to /admin/settings/createfromweb and adding the mapping manually there (title=title; isbn=isbn; authors=authors; publisher=publisher; physical_description=physical_description; body=body). I tried both with the visible labels for the fields (e.g. "Physical Description") and the machine-readable names; I double- and triple-checked to make sure that all the info was coming through and all the names really did match, but I still get that error, no matter what.

Any idea what might be causing it?