Avro schema are constant, spec are designed to be constant. As a
result, side effects are acceptable in protocol ToSpec. `assert` is
used more than usual. Wherever an inconsistent state is detected,
it's prefered to raise a argsual error message rather than unclear
exception or worse: an erroneous output.

Reference: https://avro.apache.org/docs/1.8.2/spec.html.
Any difference should be considered a bug.