diff --git a/flashcards/models.py b/flashcards/models.py
index 67b487b..39c0715 100644
--- a/flashcards/models.py
+++ b/flashcards/models.py
@@ -175,9 +175,8 @@ class User(AbstractUser, SimpleEmailConfirmationUserMixin):
         return address
 
     def by_retention(self, sections, material_date_begin, material_date_end):
-        section_pks = sections.values_list('pk')
         user_flashcard_filter = UserFlashcard.objects.filter(
-            user=self, flashcard__section__pk__in=section_pks,
+            user=self, flashcard__section__pk__in=sections,
             flashcard__material_date__gte=material_date_begin,
             flashcard__material_date__lte=material_date_end
         )
@@ -185,9 +184,7 @@ class User(AbstractUser, SimpleEmailConfirmationUserMixin):
         if not user_flashcard_filter.exists():
             raise ValidationError("No matching flashcard found in your decks")
 
-        return user_flashcard_filter.prefetch_related('userflashcardquiz_set').annotate(
-            study_count=Count('pk'),
-        ).order_by('next_review')
+        return user_flashcard_filter.order_by('next_review')
 
 
 class UserFlashcard(Model):
diff --git a/flashcards/serializers.py b/flashcards/serializers.py
index 0dba597..1a6cb5a 100644
--- a/flashcards/serializers.py
+++ b/flashcards/serializers.py
@@ -196,7 +196,7 @@ class FlashcardUpdateSerializer(serializers.Serializer):
 
 
 class QuizRequestSerializer(serializers.Serializer):
-    sections = ListField(child=IntegerField(min_value=1), required=False)
+    sections = ListField(child=IntegerField(min_value=1), required=False, default=[])
     material_date_begin = DateTimeField(default=QUARTER_START)
     material_date_end = DateTimeField(default=QUARTER_END)