Fixes: #5220 - Fix API patch request against IP-Address endpoint with null assigned_object_type

docs/release-notes/version-2.9.md

@@ -14,6 +14,7 @@
 * [#5206](https://github.com/netbox-community/netbox/issues/5206) - Apply user pagination preferences to all paginated object lists
 * [#5217](https://github.com/netbox-community/netbox/issues/5217) - Prevent erroneous removal of prefetched GenericForeignKey data from tables
 * [#5218](https://github.com/netbox-community/netbox/issues/5218) - Raise validation error if a power port's `allocated_draw` exceeds its `maximum_draw`
+* [#5220](https://github.com/netbox-community/netbox/issues/5220) - Fix API patch request against IP Address endpoint with null assigned_object_type 
 * [#5221](https://github.com/netbox-community/netbox/issues/5221) - Fix bulk component creation for virtual machines
 
 ---

netbox/ipam/api/serializers.py

@@ -219,7 +219,8 @@ class IPAddressSerializer(TaggedObjectSerializer, CustomFieldModelSerializer):
     role = ChoiceField(choices=IPAddressRoleChoices, allow_blank=True, required=False)
     assigned_object_type = ContentTypeField(
         queryset=ContentType.objects.filter(IPADDRESS_ASSIGNMENT_MODELS),
-        required=False
+        required=False,
+        allow_null=True
     )
     assigned_object = serializers.SerializerMethodField(read_only=True)
     nat_inside = NestedIPAddressSerializer(required=False, allow_null=True)