... parasite.
Not sure if that should be considered a bug. On one hand, it should
because that's core code. On the other hand, symmetry saving currently
uses parasites, which is a feature which can also be used by people
randomly. So "theoretically", there could be any data in a parasite and
we should not assume specific format.
Anyway still keep the error message but just print to standard error
output instead. Also print a bit more details (parasite name and
contents) as it would help for debugging when such a case were to occur.