Abstract The paper provides the first description of the borrowing of Croatian collective numerals into Northern Istro-Romanian and explores the consequences of this borrowing for the morphosyntax of the recipient language. It argues that the collective numerals under examination, which are specified as nominative plural feminine in the Slavic model, took on a different structural specification in the Romance replica, in a way that led to a restructuring of the morphosyntactic system, introducing (sub)gender overdifferentiation on just two agreement targets and, thereby, a complexification in this area of grammar. The illustration of this change is placed against the background of the other contact-induced changes that grammatical gender has undergone in Istro-Romanian during the 20th century, which have led to the borrowing of two dedicated forms in distinct inflectional cells and the rise of two separate defective gender values, each the replica of one number value of the Slavic neuter.