The error message in src/mongo/db/matcher/schema/encrypt_schema_types.cpp#L44-L46 is very vague. In particular, this is confusing because (at least with the Node.js libmongocrypt bindings, but likely also the others) the error shows up when actually trying to insert documents using this schema, rather than when the schema is first specified. This makes it seem like something is wrong with the inserted document, rather than the schema itself.

I will open a pull request with a suggestion.