Cury and Krogh have formulated a robust supervisory control problem to synthesize a supervisor for the nominal plant which maximizes robustness. They have solved the problem by unnecessarily restricting the upper bound of the legal behavior. In this note, we show that the problem can be solved without restricting the upper hound of the legal behavior when the specification is described by prefix-closed languages. We synthesize a maximally permissive supervisor for the nominal plant which maximizes not only robustness, but also permissiveness for the maximal set of admissible plant variations.