A technique for the synthesis of supervisory controllers for discrete-event systems, modeled as Petri-nets with uncontrollable transitions, is proposed. Based on the concept of place invariants, the supervisor is designed to enforce the respect of a set of operation constraints. Due to the uncontrollable transitions, it may become impossible to directly enforce the respect of a given constraint, which is then said to be inadmissible. It is then proposed a method to determine a set of admissible constraints which do not violate the original constraint. By obtaining a larger number of admissible constraints, the proposed method is able to derive more permissive controllers than those proposed in the literature. Its effectiveness is illustrated by an example.