diff --git a/django/db/models/query.py b/django/db/models/query.py
index fea1144200..5c7d08fb39 100644
--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -796,7 +796,7 @@ class QuerySet(object):
         Returns the internal query's SQL and parameters (as a tuple).
         """
         obj = self.values("pk")
-        if connection == connections[obj.db]:
+        if obj._db is None or connection == connections[obj._db]:
             return obj.query.get_compiler(connection=connection).as_nested_sql()
         raise ValueError("Can't do subqueries with queries on different DBs.")
 
@@ -918,7 +918,7 @@ class ValuesQuerySet(QuerySet):
                     % self.__class__.__name__)
 
         obj = self._clone()
-        if connection == connections[obj.db]:
+        if obj._db is None or connection == connections[obj._db]:
             return obj.query.get_compiler(connection=connection).as_nested_sql()
         raise ValueError("Can't do subqueries with queries on different DBs.")
 
diff --git a/tests/regressiontests/multiple_database/tests.py b/tests/regressiontests/multiple_database/tests.py
index 81b6a5ffb9..540648f0e8 100644
--- a/tests/regressiontests/multiple_database/tests.py
+++ b/tests/regressiontests/multiple_database/tests.py
@@ -655,6 +655,19 @@ class QueryTestCase(TestCase):
         # The editor instance should have a db state
         self.assertEqual(book.editor._state.db, 'other')
 
+    def test_subquery(self):
+        """Make sure as_sql works with subqueries and master/slave."""
+        sub = Person.objects.using('other').filter(name='fff')
+        qs = Book.objects.filter(editor__in=sub)
+
+        # When you call __str__ on the query object, it doesn't know about using
+        # so it falls back to the default. If the subquery explicitly uses a
+        # different database, an error should be raised.
+        self.assertRaises(ValueError, str, qs.query)
+
+        # Evaluating the query shouldn't work, either
+        self.assertRaises(ValueError, list, qs)
+
 class TestRouter(object):
     # A test router. The behaviour is vaguely master/slave, but the
     # databases aren't assumed to propagate changes.
@@ -1137,6 +1150,26 @@ class RouterTestCase(TestCase):
         review3.content_object = dive
         self.assertEquals(review3._state.db, 'default')
 
+    def test_subquery(self):
+        """Make sure as_sql works with subqueries and master/slave."""
+        # Create a book and author on the other database
+
+        mark = Person.objects.using('other').create(name="Mark Pilgrim")
+        dive = Book.objects.using('other').create(title="Dive into Python",
+                                                  published=datetime.date(2009, 5, 4),
+                                                  editor=mark)
+
+        sub = Person.objects.filter(name='Mark Pilgrim')
+        qs = Book.objects.filter(editor__in=sub)
+
+        # When you call __str__ on the query object, it doesn't know about using
+        # so it falls back to the default. Don't let routing instructions
+        # force the subquery to an incompatible database.
+        str(qs.query)
+
+        # If you evaluate the query, it should work, running on 'other'
+        self.assertEquals(list(qs.values_list('title', flat=True)), [u'Dive into Python'])
+
 class AuthTestCase(TestCase):
     multi_db = True