Mathematical modelling can be described as "[…] the process of translating between the real world and mathematics in both directions" (Blum & Ferri, 2009, p. 45). Both students and teachers often find mathematical modelling challenging. Thus, it is important that future teachers are aware of the challenges their students might encounter when working on this topic. Based on observations and interviews, this study examines obstacles that Norwegian pre-service teachers face in mathematical modelling, and how they perceive their own solution process after learning about mathematical modelling. The study draws on a seven-step modelling cycle described by Blum (2015), and reveals that the pre-service teachers, due to the lack of assumptions, struggle to give meaningful interpretations of their mathematical results. Their perceptions about having to provide their answer as a general expression, are found to be related to their expectations stemming from classroom activities different from student-centered modelling sessions.