diff --git a/mistral/cmd/launch.py b/mistral/cmd/launch.py
index c965bd20c..595de850e 100644
--- a/mistral/cmd/launch.py
+++ b/mistral/cmd/launch.py
@@ -194,7 +194,7 @@ def main():
             # Validate launch option.
             if set(cfg.CONF.server) - set(LAUNCH_OPTIONS.keys()):
                 raise Exception('Valid options are all or any combination of '
-                                'api, engine, and executor.')
+                                ', '.join(LAUNCH_OPTIONS.keys()))
 
             # Launch distinct set of server(s).
             launch_any(set(cfg.CONF.server))