Just for completeness's sake, I discovered that my problem was that I hadn't included some of my referenced scalar transformers in the map that I passed, so the spec failure was "expecting a spec, but found nil instead"

PEBKAC again

We expect you to pass a very, very complex data structure into compile, and great as clojure.spec is, you can easily get lost in its description of the failure. So I think we have a bit of unwanted/unavoidable complexity here, not ID10T errors.