diff --git a/netbox/extras/signals.py b/netbox/extras/signals.py
index ed3da7bfd..2813ed7ae 100644
--- a/netbox/extras/signals.py
+++ b/netbox/extras/signals.py
@@ -2,7 +2,7 @@ import importlib
 import logging
 
 from django.contrib.contenttypes.models import ContentType
-from django.core.exceptions import ValidationError
+from django.core.exceptions import ImproperlyConfigured, ValidationError
 from django.db.models.fields.reverse_related import ManyToManyRel
 from django.db.models.signals import m2m_changed, post_save, pre_delete
 from django.dispatch import receiver, Signal
@@ -41,6 +41,9 @@ def run_validators(instance, validators):
         elif type(validator) is dict:
             validator = CustomValidator(validator)
 
+        elif not issubclass(validator.__class__, CustomValidator):
+            raise ImproperlyConfigured(f"Invalid value for custom validator: {validator}")
+
         validator(instance, request)