https://hg.python.org/cpython/file/2df7c958974e/Objects/dictobject.c#l451

The comments near lookdict suggest that specialized versions such as lookdict_unicode and lookdict_unicode_nodummy cannot return NULL, as that would indicate an Exception was raised during comparison.

They can return NULL, because if the *search* key is not unicode, they replace themselves with the generic lookdict and then return its result, which may be NULL.