Where Does Nuclear Energy Come From?

Physics students in their examination answers often write that "nuclear fission releases energy because mass is converted into energy" .  At a first glance this statement may seem plausible due to Einstein's well known equation. However, upon closer consideration, based on remembering that the protons and neutrons are rearranged into different combinations in the reaction and so their total number does not change, where does the mass that is converted into energy come from?