diff --git a/nova/compute/resource_tracker.py b/nova/compute/resource_tracker.py
index e903acc5660b..2c3e15496870 100644
--- a/nova/compute/resource_tracker.py
+++ b/nova/compute/resource_tracker.py
@@ -1114,8 +1114,14 @@ class ResourceTracker(object):
             itype = self._get_instance_type(instance, 'old_', migration)
             numa_topology = self._get_migration_context_resource(
                 'numa_topology', instance, prefix='old_')
-            LOG.debug('Starting to track outgoing migration %s with flavor %s',
-                      migration.uuid, itype.flavorid, instance=instance)
+            # We could be racing with confirm_resize setting the
+            # instance.old_flavor field to None before the migration status
+            # is "confirmed" so if we did not find the flavor in the outgoing
+            # resized instance we won't track it.
+            if itype:
+                LOG.debug('Starting to track outgoing migration %s with '
+                          'flavor %s', migration.uuid, itype.flavorid,
+                          instance=instance)
 
         if itype:
             cn = self.compute_nodes[nodename]
diff --git a/nova/tests/unit/compute/test_resource_tracker.py b/nova/tests/unit/compute/test_resource_tracker.py
index 17943e0484a4..f6ed4e5b8323 100644
--- a/nova/tests/unit/compute/test_resource_tracker.py
+++ b/nova/tests/unit/compute/test_resource_tracker.py
@@ -2761,6 +2761,29 @@ class TestUpdateUsageFromMigration(test.NoDBTestCase):
                                         _NODENAME)
         self.assertFalse(get_mock.called)
 
+    def test_missing_old_flavor_outbound_resize(self):
+        """Tests the case that an instance is not being tracked on the source
+        host because it has been resized to a dest host. The confirm_resize
+        operation in ComputeManager sets instance.old_flavor to None before
+        the migration.status is changed to "confirmed" so the source compute
+        RT considers it an in-progress migration and tries to update tracked
+        usage from the instance.old_flavor (which is None when
+        _update_usage_from_migration runs). This test just makes sure that the
+        RT method gracefully handles the instance.old_flavor being gone.
+        """
+        migration = _MIGRATION_FIXTURES['source-only']
+        rt = resource_tracker.ResourceTracker(
+            migration.source_compute, mock.sentinel.virt_driver)
+        ctxt = context.get_admin_context()
+        instance = objects.Instance(
+            uuid=migration.instance_uuid, old_flavor=None,
+            migration_context=objects.MigrationContext())
+        rt._update_usage_from_migration(
+            ctxt, instance, migration, migration.source_node)
+        self.assertNotIn('Starting to track outgoing migration',
+                         self.stdlog.logger.output)
+        self.assertNotIn(migration.instance_uuid, rt.tracked_migrations)
+
 
 class TestUpdateUsageFromMigrations(BaseTestCase):
     @mock.patch('nova.compute.resource_tracker.ResourceTracker.'