diff --git a/commands.py b/commands.py
index d0bae43..4200759 100644
--- a/commands.py
+++ b/commands.py
@@ -115,7 +115,7 @@ def frappy_start(**services):
             if cfg:
                 prev = used_cfg.get(cfg)
                 if prev:
-                    raise ValueError('%r can not be used in both %s and %s' % (prev, service))
+                    raise ValueError('%r can not be used in both %s and %s' % (cfg, prev, service))
                 used_cfg[cfg] = service
 
     if new_cfg: