diff --git a/taiga/base/filters.py b/taiga/base/filters.py
index f8840111..154a074b 100644
--- a/taiga/base/filters.py
+++ b/taiga/base/filters.py
@@ -22,7 +22,7 @@ from dateutil.parser import parse as parse_date
 
 from django.apps import apps
 from django.contrib.contenttypes.models import ContentType
-from django.db.models import Q
+from django.db.models import Q, OuterRef, Subquery
 from django.utils.translation import ugettext as _
 
 from taiga.base import exceptions as exc
@@ -423,24 +423,19 @@ class AssignedToFilter(BaseRelatedFieldsFilter):
 class AssignedUsersFilter(BaseRelatedFieldsFilter):
     filter_name = 'assigned_users'
 
-    def get_lookup_expression(self, field_name, value):
-        if None in value:
-            qs_in_kwargs = {
-                "{}__in".format(field_name): [v for v in value if
-                                                    v is not None]}
-            qs_isnull_kwargs = {"{}__isnull".format(field_name): True}
-            return Q(**qs_in_kwargs) | Q(**qs_isnull_kwargs)
-        else:
-            return Q(**{"{}__in".format(field_name): value})
-
     def _get_queryparams(self, params):
         param_name = self.param_name or self.filter_name
         raw_value = params.get(param_name, None)
 
         if raw_value:
             value = self._prepare_filter_data(raw_value)
-            assigned_user_filter = self.get_lookup_expression(param_name, value)
-            assigned_to_filter = self.get_lookup_expression('assigned_to', value)
+            UserStoryModel = apps.get_model("userstories", "UserStory")
+
+            assigned_users_ids = UserStoryModel.objects.order_by().filter(
+                assigned_users__in=value, id=OuterRef('pk')).values('pk')
+
+            assigned_user_filter = Q(pk__in=Subquery(assigned_users_ids))
+            assigned_to_filter = Q(assigned_to__in=value)
 
             return Q(assigned_user_filter | assigned_to_filter)