The article applies computational stylometry to explore medieval authorship. The study consists of the investigation of different versions of an anti-heretical treatise Refutatio errorum, only recently attributed to the inquisitor Petrus Zwicker based on qualitative evidence. The article confirms this attribution and demonstrates that from a textually inconsistent corpus, it is possible to prepare data for computational stylometry with relatively fast and straightforward cleaning. Finally, the article discusses the relationship between qualitative and computational methods in the study of medieval texts. The greatest added value of computational authorship attributions comes from unexpected results, from texts behaving in an anomalous way. In the classifications made for this study, a small work called Attendite a falsis prophetis was for the first time attributed to Petrus Zwicker, a prime example of an unanticipated result. This attribution is within possibilities, but its final corroboration requires codicological study of the preserved manuscripts.