System developers spend a significant part of their time debugging systems (i.e., locating and fixing the cause of failures observed through verification and validation (V&V)). While V&V techniques are commonly used in model-driven engineering, locating and fixing the cause of a failure in a modelled system is most often still a manual task without tool-support. Although debugging techniques are well-established for programming languages, only a few debugging techniques and tools for models have been proposed. Debugging models faces various challenges: handling a wide variety of models and modelling languages; adapting debugging techniques initially proposed for programming languages; tailoring debugging approaches for the domain expert using the abstractions of the considered language. The aim of the first edition of the MDEbug workshop was to bring together researchers wanting to contribute to the emerging field of debugging in model-driven engineering by discussing new ideas and compiling a research agenda. This paper summarizes the workshop's discussion session and distils a list of challenges that should be addressed in future research.