One of the most compelling pieces of evidence for the presence of predictions in language comprehension comes from event-related potential (ERP) studies which show that encountering an adjective whose gender marking is inconsistent with that of a highly expectable noun leads to an effect at the adjective. Until now the mechanism underlying this effect has been unknown. The present study tests a novel hypothesis whereby the effect at adjectives reflects prediction updating, which in turn impacts the N400 generated by the upcoming noun. In 2 experiments, native speakers of Polish read short stories that made several nouns likely at the position of a direct object in the story's final sentence. The noun was preceded by a gender-marked adjective which was shown in either of the 2 grammatical forms: (a) informative about the following noun (adjectival gender marking constrained which nouns can follow), or (b) less informative (the grammatical marking did not disambiguate the gender of the following noun). Importantly, the same stories and critical words were used in both conditions, which was possible thanks to exploiting syncretisms in Polish adjectival inflection. The results provide support for 1 or more mechanisms of prediction updating at the adjective that impact the amplitude of the N400 elicited by the following noun. Together, these results show that when only morphosyntactic cues are available, they are instantly used to update predictions about upcoming words.