Thanks, Jürgen, being less of a doofus with the contract definition seems to have helped.
?
(I had seen that documentation -- I stand by my opinion that it could be better documented; I think that "Uncommonly received" is a less helpful message than, say, "if this occurs repeatedly, check you are sending a properly-defined contract". In any case, I hope future people hitting the same problem[^0] will find this thread and find it helpful.)
[^0]: Even if it's just me, I'm ok with that.
