Nil-filing refers to taxpayers who report zero on all fields of their tax declaration. It is a largely ignored phenomenon in the tax literature, despite being well known to tax administrators. There is almost no evidence on the characteristics of nil-filers and the reasons for their apparently puzzling behaviour. This paper sheds light on this issue in Rwanda, using a descriptive analysis of administrative data, a randomised controlled trial (RCT), and qualitative interviews with taxpayers and tax officials. We argue that evasion is part of the explanation for nil-filing, but it seems to play a relatively small role. Instead, a major reason for nil-filing lies at the interaction between aggressive recruitment campaigns by the Rwanda Revenue Authority (RRA), and taxpayers’ response to a complex and often confusing tax system.