diff --git a/watcher/common/exception.py b/watcher/common/exception.py
index 12f952c87..7326ffba4 100644
--- a/watcher/common/exception.py
+++ b/watcher/common/exception.py
@@ -305,7 +305,7 @@ class ActionFilterCombinationProhibited(Invalid):
 
 
 class UnsupportedActionType(UnsupportedError):
-    msg_fmt = _("Provided %(action_type) is not supported yet")
+    msg_fmt = _("Provided %(action_type)s is not supported yet")
 
 
 class EfficacyIndicatorNotFound(ResourceNotFound):