Good idea. I checked all alphanumeric characters whose UTF-8 encoding ends with \xa1. >>> '\N{fullwidth latin capital letter a}'.encode('utf-8') b'\xef\xbc

Good idea. I checked all alphanumeric characters whose UTF-8 encoding ends with \xa1. >>> '\N{fullwidth latin capital letter a}'.encode('utf-8') b'\xef\xbc\xa1' Ａ seems like the culprit.