In this paper we analyze the influence of Guarani, a genderless language, on patterns of gender agreement in two distinct Spanish varieties, Paraguayan Spanish and Correntino Spanish. Using two unique conversational corpora, we show that, of the two varieties, Paraguayan Spanish more closely resembles Standard Spanish in patterns of gender agreement, but the cases of nonagreement that do occur are found in a wide variety of linguistic contexts, resembling the kinds of transfer effects common in L2 varieties of Spanish in speakers whose L1 is genderless. Correntino Spanish exhibits patterns of nonagreement that are not only more common but are limited in linguistic scope, i.e., only occurring in noun phrases involving feminine nouns with non-article modifiers. We attribute the differences between these two varieties to the differing presence of Guarani in the two regions where they are currently spoken. Paraguayan Spanish, in closer contact with Guarani, is subject to synchronic L2 transfer effects, while Correntino Spanish, a largely monolingual variety in the modern day, is characterized by diachronic L2 transfer effects that have fossilized and come to be found in the speech of monolinguals.