The assertion was valid while all keywords did came from a constant keywords tuple in CALL_FUNCTION_KW. But if the FASTCALL protocol is extended for var-keyword arguments, keywords can be arbitrary and the assertion can fail. The assertion on the next line can fail too.
Calling _PyStack_AsDict() with non-string or non-unique keywords means that the var-keyword argument was first unpacked to arrays of keyword names and values and then converted back to a dict. Seems something is done non-efficiently.