diff --git a/error.py b/error.py
index 48e3189..783ab7d 100644
--- a/error.py
+++ b/error.py
@@ -85,8 +85,8 @@
      repo or manifest repositories.  In this special case we must
      use exec to re-execute repo with the new code and manifest.
   """
-  def __init__(self, extra_args=[]):
-    self.extra_args = extra_args
+  def __init__(self, extra_args=None):
+    self.extra_args = extra_args or []
 
 class HookError(Exception):
   """Thrown if a 'repo-hook' could not be run.