diff -r ebfd1b96a013 -r b922d144f93d hgext/obsolete.py
--- a/hgext/obsolete.py	Wed Jul 04 18:00:43 2012 +0200
+++ b/hgext/obsolete.py	Fri Jul 06 15:39:01 2012 +0200
@@ -942,6 +942,8 @@
     if not repo.local():
         return
 
+    if not util.safehasattr(repo.opener, 'tryread'):
+        raise util.Abort('Obsolete extension require Mercurial 2.2 (or later)')
     opull = repo.pull
     opush = repo.push
     olock = repo.lock