diff --git a/machfs/directory.py b/machfs/directory.py
index b070f90..85e7391 100644
--- a/machfs/directory.py
+++ b/machfs/directory.py
@@ -16,7 +16,7 @@ class AbstractFolder(collections.MutableMapping):
         key.encode('mac_roman')
 
         if not (1 <= len(key) <= 31):
-            raise ValueError('Filename range 1-31 chars')
+            raise ValueError('%r: Filename range 1-31 chars' % key)
 
         lower = key.lower()
         self._prefdict[lower] = key