diff --git a/plumpy/base/state_machine.py b/plumpy/base/state_machine.py
index a3996d5a..e90cb827 100644
--- a/plumpy/base/state_machine.py
+++ b/plumpy/base/state_machine.py
@@ -286,6 +286,9 @@ def add_state_event_callback(self, hook: Hashable, callback: EVENT_CALLBACK_TYPE
         self._event_callbacks.setdefault(hook, []).append(callback)
 
     def remove_state_event_callback(self, hook: Hashable, callback: EVENT_CALLBACK_TYPE) -> None:
+        if getattr(self, '_closed', False):
+            # if the process is closed, then all callbacks have already been removed
+            return None
         try:
             self._event_callbacks[hook].remove(callback)
         except (KeyError, ValueError):