diff --git a/netbox/extras/models/customfields.py b/netbox/extras/models/customfields.py
index 1f6975c73d..73d5cf9ce6 100644
--- a/netbox/extras/models/customfields.py
+++ b/netbox/extras/models/customfields.py
@@ -313,9 +313,10 @@ def serialize(self, value):
         """
         if value is None:
             return value
-        if self.type in (CustomFieldTypeChoices.TYPE_DATE, CustomFieldTypeChoices.TYPE_DATETIME):
-            if type(value) in (date, datetime):
-                return value.isoformat()
+        if self.type == CustomFieldTypeChoices.TYPE_DATE and type(value) is date:
+            return value.isoformat()
+        if self.type == CustomFieldTypeChoices.TYPE_DATETIME and type(value) is datetime:
+            return value.isoformat()
         if self.type == CustomFieldTypeChoices.TYPE_OBJECT:
             return value.pk
         if self.type == CustomFieldTypeChoices.TYPE_MULTIOBJECT: