diff --git a/nova/exception.py b/nova/exception.py
index 7f8ff0db0e..274acaa88a 100644
--- a/nova/exception.py
+++ b/nova/exception.py
@@ -894,7 +894,7 @@ class CellError(NovaException):
 
 
 class CellsUpdateUnsupported(NovaException):
-    message = _("Cannot update cells configuration file.")
+    msg_fmt = _("Cannot update cells configuration file.")
 
 
 class InstanceUnknownCell(NotFound):