diff --git a/src/tixlbase/cache.py b/src/tixlbase/cache.py
index f6099d4b39..0cce7258bd 100644
--- a/src/tixlbase/cache.py
+++ b/src/tixlbase/cache.py
@@ -21,6 +21,7 @@ class EventRelatedCache:
     """
 
     def __init__(self, event: Event, cache: str='default'):
+        assert isinstance(event, Event)
         self.cache = caches[cache]
         self.event = event
         self.prefixkey = 'event:%s' % self.event.pk
diff --git a/src/tixlbase/signals.py b/src/tixlbase/signals.py
index 36603fdd40..b008917aa6 100644
--- a/src/tixlbase/signals.py
+++ b/src/tixlbase/signals.py
@@ -2,6 +2,8 @@ import django.dispatch
 from django.apps import apps
 from django.dispatch.dispatcher import NO_RECEIVERS
 
+from tixlbase.models import Event
+
 
 class EventPluginSignal(django.dispatch.Signal):
     """
@@ -17,6 +19,8 @@ class EventPluginSignal(django.dispatch.Signal):
 
         sender is required to be an instance of ``tixlbase.models.Event``.
         """
+        assert isinstance(sender, Event)
+
         responses = []
         if not self.receivers or self.sender_receivers_cache.get(sender) is NO_RECEIVERS:
             return responses
diff --git a/src/tixlcontrol/views/forms.py b/src/tixlcontrol/views/forms.py
index 52696b8751..2a569205ae 100644
--- a/src/tixlcontrol/views/forms.py
+++ b/src/tixlcontrol/views/forms.py
@@ -258,6 +258,7 @@ class VariationsField(forms.ModelMultipleChoiceField):
         super().__init__(*args, **kwargs)
 
     def set_item(self, item: Item):
+        assert isinstance(item, Item)
         self.item = item
         self._set_choices(self._get_choices())