I am seeing a triggering of one of the assertions in convert_to_error, in managers.py. (This assertion is completely without any message, incidentally, which is also problematic - I've commented on this in issue5001.) This is in common across all versions of cpython that I have access to, and possibly to pypy as well. See https://travis-ci.org/drallensmith/neat-python/builds/262839326 for a set of examples. (The assertion is that "result" is a string. It is somewhat problematic when error-reporting code has an error...)