diff --git a/tools-clean.py b/tools-clean.py
index 6344b15..bbfa01f 100644
--- a/tools-clean.py
+++ b/tools-clean.py
@@ -11,7 +11,7 @@
         encoding["labels"] = labels
         for label in labels:
             if label in labelsseen:
-                raise("Duplicate label: ", label)
+                raise ValueError("Duplicate label: {!r}".format(label))
             labelsseen.append(label)
 
 handle = open(filename, "w")