Index: django/db/models/sql/query.py
===================================================================
--- django/db/models/sql/query.py	(revision 9781)
+++ django/db/models/sql/query.py	(working copy)
@@ -295,10 +295,14 @@
         query.related_select_cols = []
         query.related_select_fields = []
 
+        result = query.execute_sql(SINGLE)
+        if result is None:
+            return {None: 0}
+        
         return dict([
             (alias, self.resolve_aggregate(val, aggregate))
             for (alias, aggregate), val
-            in zip(query.aggregate_select.items(), query.execute_sql(SINGLE))
+                in zip(query.aggregate_select.items(), result)
         ])
 
     def get_count(self):

Index: tests/regressiontests/aggregation_regress/models.py
===================================================================
--- tests/regressiontests/aggregation_regress/models.py	(revision 9781)
+++ tests/regressiontests/aggregation_regress/models.py	(working copy)
@@ -151,6 +151,10 @@
 >>> Book.objects.select_related('publisher').annotate(num_authors=Count('authors')).values()[0]
 {'rating': 4.0, 'isbn': u'013790395', 'name': u'Artificial Intelligence: A Modern Approach', 'pubdate': datetime.date(1995, 1, 15), 'price': Decimal("82.8..."), 'id': 5, 'num_authors': 2, 'publisher_id': 3, 'pages': 1132}
 
+# Regression for #10089: count() with empty QuerySet
+>>> Book.objects.filter(id__in=[]).count()
+0
+
 """
 }