diff --git a/src/django_rest_aggregation/aggregator.py b/src/django_rest_aggregation/aggregator.py
index 2a458fc..91431c8 100644
--- a/src/django_rest_aggregation/aggregator.py
+++ b/src/django_rest_aggregation/aggregator.py
@@ -1,5 +1,6 @@
 from django.core.exceptions import FieldDoesNotExist
 from django.db import models
+from django.db.models import Case, When, Value
 from rest_framework.exceptions import ValidationError
 from rest_framework.request import Request
 
@@ -83,7 +84,9 @@ def get_aggregated_queryset(self):
         self.validate_params()
 
         if (group_by := self.params.get("group_by", ["group"])) == ["group"]:
-            self.queryset = self.queryset.annotate(group=models.Value("all", output_field=models.CharField()))
+            self.queryset = self.queryset.annotate(group=Case(
+                When(pk__isnull=False, then=Value("all")),
+                default=Value("test")))
 
         return self.queryset.values(*group_by).annotate(**get_annotation(self.params, self.aggregation_name))
 
diff --git a/src/django_rest_aggregation/mixins.py b/src/django_rest_aggregation/mixins.py
index 4ada77f..6d4e157 100644
--- a/src/django_rest_aggregation/mixins.py
+++ b/src/django_rest_aggregation/mixins.py
@@ -10,7 +10,7 @@
 class AggregationMixin:
     @action(methods=["get"], detail=False, url_path="aggregation", url_name="aggregation")
     def aggregation(self, request):
-        queryset = self.get_queryset()
+        queryset = self.get_queryset().order_by()
 
         if DjangoFilterBackend in self.filter_backends:
             queryset = DjangoFilterBackend().filter_queryset(request, queryset, self)
@@ -43,12 +43,12 @@ def get_aggregation_name(self):
 
     def filter_aggregated_queryset(self, queryset):
         ordering_fields = getattr(self, "ordering_fields", [])
-        valid_fields = queryset[0].keys()
+        valid_fields = queryset[0].keys() if queryset.exists() else []
 
         if ordering_fields == "__all__":
             ordering_fields = valid_fields
         else:
-            ordering_fields = list(set(ordering_fields).intersection(set(queryset[0].keys())))
+            ordering_fields = list(set(ordering_fields).intersection(set(valid_fields)))
 
         if (fields := getattr(self, "aggregated_filterset_fields", None)) is not None:
             ValueFilter.set_filter_fields(fields, self.get_aggregation_name())