Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† prŏlŏgĭum, ĭi, n., = προλόγιον, a preface, introduction: prologium, principium, proloquium. Pacuvius: quid est? nam me exanimasti prologio tuo, Paul. ex Fest. p 226 Müll.