diff --git a/Lib/locale.py b/Lib/locale.py
index f6d174c..4ab3c6a 100644
--- a/Lib/locale.py
+++ b/Lib/locale.py
@@ -121,12 +121,15 @@
 
 # Iterate over grouping intervals
 def _grouping_intervals(grouping):
+    last_interval = None
     for interval in grouping:
         # if grouping is -1, we are done
         if interval == CHAR_MAX:
             return
         # 0: re-use last group ad infinitum
         if interval == 0:
+            if last_interval is None:
+                raise ValueError("invalid grouping")
             while True:
                 yield last_interval
         yield interval