How does cross-linguistic variation in grammatical structure affect children’s acquisition of number words? In this study, we addressed this question by investigating the case study of young speakers of French, a language in which the number one and the indefinite article a are phonologically the same (i.e., un). We tested how French-speaking children interpret un, and whether it more closely resembles the English word a or one. We found that French-speaking children almost always accepted sets of 1 for un, but that their responses for sets of 2 were more equivocal, with many children saying “Oui” (Yes) when asked whether there was un. Overall, French children’s interpretation of un differed from how English-speaking interpret both a and one. This suggests that French-speaking children’s interpretation of un reflects the ambiguity of the input that they are exposed to. We conclude that French morphological structure may pose a challenge to French- speaking children in acquiring an exact numerical meaning for the word un, potentially causing a delay in number word learning.