diff --git a/subcmds/download.py b/subcmds/download.py
index a6f3aa4..61eadd5 100644
--- a/subcmds/download.py
+++ b/subcmds/download.py
@@ -36,6 +36,9 @@
     pass
 
   def _ParseChangeIds(self, args):
+    if not args:
+      self.Usage()
+
     to_get = []
     project = None