<p>In hierarchical supervisory control of discrete event systems, the observer property of the reporter map ensures that the high-level supervisor can rely on the occurrence of any event that is eligible in the high-level plant to implement a nonblocking closed-loop system. When combined with strict output control consistency and marking consistency, the observer property is a sufficient but not necessary condition for strong hierarchical consistency, which guarantees that the high-level model maintains the necessary information to achieve optimal nonblocking supervisory control for any high-level specification. Aiming to weaken the observer condition, this paper introduces the definitions of prefix-closing alphabet and of reliable events. The first main result demonstrates that it is sufficient to verify if the subset of reliable events is prefix-closing for the high-level controllable specification. A second result proves that nonblocking hierarchical supervision is ensured for any high-level specification if the set of uncontrollable high-level events is reliable and prefix-closing for the high-level plant. The application to the hierarchical structure of a valve circuit in process control where the reporter map is not an observer demonstrates that the novel conditions can be useful in practice.</p>