diff --git a/slac_timing/event_definition.py b/slac_timing/event_definition.py
index 4ec7434..86b7a6c 100644
--- a/slac_timing/event_definition.py
+++ b/slac_timing/event_definition.py
@@ -57,7 +57,7 @@ def _reserve(self) -> int:
                         f"Could not reach edef system pv={_SYSTEM.slot_names[num].pvname}"
                     )
                 if edef_name == self.name:
-                    if not _SYSTEM.slot_usernames[num].put(str(self.user), wait=True):
+                    if _SYSTEM.slot_usernames[num].put(str(self.user), wait=True) is None:
                         raise ReservationError(
                             f"Could not reach edef system pv={_SYSTEM.slot_usernames[num].pvname}"
                         )