diff --git a/django/db/models/sql/aggregates.py b/django/db/models/sql/aggregates.py
index 3cda4d244e..0e1794e0a9 100644
--- a/django/db/models/sql/aggregates.py
+++ b/django/db/models/sql/aggregates.py
@@ -79,7 +79,7 @@ class Aggregate(object):
         elif isinstance(self.col, (list, tuple)):
             field_name = '.'.join(qn(c) for c in self.col)
         else:
-            field_name = self.col
+            field_name = qn(self.col)
 
         substitutions = {
             'function': self.sql_function,
diff --git a/tests/aggregation_regress/tests.py b/tests/aggregation_regress/tests.py
index b8ec14d1dc..741e2ed734 100644
--- a/tests/aggregation_regress/tests.py
+++ b/tests/aggregation_regress/tests.py
@@ -1114,3 +1114,13 @@ class AggregationTests(TestCase):
         self.assertQuerysetEqual(
             qs, ['Sams Teach Yourself Django in 24 Hours'],
             lambda b: b.name)
+
+    def test_annotate_reserved_word(self):
+        """
+        Regression #18333 - Ensure annotated column name is properly quoted.
+        """
+        vals = Book.objects.annotate(select=Count('authors__id')).aggregate(Sum('select'), Avg('select'))
+        self.assertEqual(vals, {
+            'select__sum': 10,
+            'select__avg': Approximate(1.666, places=2),
+        })