[en] We describe how the linguistic theory of presuppositions can be used to analyse and model the usage of partial functions and undefined terms in mathematical texts. We compare our account to other accounts of partial functions and undefined terms, showing how our account models the actual usage of partial functions and undefined terms more faithfully than existing accounts. The model described in this paper has been developed for the Naproche system, a computer system for proof-checking mathematical texts written in controlled natural language, and has largely been implemented in this system.