Fixes #5331: Fix filtering of sites by null region

netbox/utilities/filters.py

@@ -68,11 +68,10 @@ class TreeNodeMultipleChoiceFilter(django_filters.ModelMultipleChoiceFilter):
     """
     Filters for a set of Models, including all descendant models within a Tree.  Example: [<Region: R1>,<Region: R2>]
     """
-
     def get_filter_predicate(self, v):
-        # null value filtering
+        # Null value filtering
         if v is None:
-            return {self.field_name.replace('in', 'isnull'): True}
+            return {f"{self.field_name}__isnull": True}
         return super().get_filter_predicate(v)
 
     def filter(self, qs, value):

netbox/utilities/tests/test_filters.py

@@ -23,7 +23,8 @@ class TreeNodeMultipleChoiceFilterTest(TestCase):
     class SiteFilterSet(django_filters.FilterSet):
         region = TreeNodeMultipleChoiceFilter(
             queryset=Region.objects.all(),
-            field_name='region__in',
+            field_name='region',
+            lookup_expr='in',
             to_field_name='slug',
         )