diff --git a/nova/exception.py b/nova/exception.py
index ac035ef..8787526 100644
--- a/nova/exception.py
+++ b/nova/exception.py
@@ -865,6 +865,10 @@ class CannotResizeToSameFlavor(NovaException):
     message = _("When resizing, instances must change flavor!")
 
 
+class ResizeError(NovaException):
+    message = _("Resize error") + ": %(reason)s"
+
+
 class ImageTooLarge(NovaException):
     message = _("Image is larger than instance type allows")
 
diff --git a/nova/tests/test_xenapi.py b/nova/tests/test_xenapi.py
index 5253d62..cf970e4 100644
--- a/nova/tests/test_xenapi.py
+++ b/nova/tests/test_xenapi.py
@@ -1254,6 +1254,20 @@ class XenAPIMigrateInstance(stubs.XenAPITestBase):
                               dict(base_copy='hurr', cow='durr'),
                               network_info, image_meta, resize_instance=False)
 
+    def test_migrate_no_auto_disk_config_no_resize_down(self):
+        """Resize down should fail when auto_disk_config not set"""
+        instance_values = self.instance_values
+        instance_values['root_gb'] = 40
+        instance_values['auto_disk_config'] = False
+        instance = db.instance_create(self.context, instance_values)
+        xenapi_fake.create_vm(instance.name, 'Running')
+        instance_type = db.instance_type_get_by_name(self.context, 'm1.small')
+        conn = xenapi_conn.XenAPIDriver(fake.FakeVirtAPI(), False)
+        self.assertRaises(exception.ResizeError,
+                          conn.migrate_disk_and_power_off,
+                          self.context, instance,
+                          '127.0.0.1', instance_type, None)
+
 
 class XenAPIImageTypeTestCase(test.TestCase):
     """Test ImageType class."""
diff --git a/nova/virt/xenapi/vmops.py b/nova/virt/xenapi/vmops.py
index 4915804..a68c9eb 100644
--- a/nova/virt/xenapi/vmops.py
+++ b/nova/virt/xenapi/vmops.py
@@ -770,8 +770,10 @@ class VMOps(object):
         """
         vm_ref = self._get_vm_opaque_ref(instance)
         sr_path = vm_utils.get_sr_path(self._session)
-        resize_down = (instance['auto_disk_config'] and
-                       instance['root_gb'] > instance_type['root_gb'])
+        resize_down = instance['root_gb'] > instance_type['root_gb']
+        if resize_down and not instance['auto_disk_config']:
+            reason = _('Resize down not allowed without auto_disk_config')
+            raise exception.ResizeError(reason=reason)
 
         # 0. Zero out the progress to begin
         self._update_instance_progress(context, instance,