diff -r 926bc0d3b595 -r 650b5b6e75ed mercurial/cmdutil.py
--- a/mercurial/cmdutil.py	Wed Aug 27 12:30:28 2014 +0200
+++ b/mercurial/cmdutil.py	Tue Aug 26 22:03:32 2014 +0200
@@ -2130,7 +2130,7 @@
                                                   copied=copied.get(path))
                         return mctx
                     except KeyError:
-                        raise IOError
+                        return None
             else:
                 ui.note(_('copying changeset %s to %s\n') % (old, base))
 
@@ -2139,7 +2139,7 @@
                     try:
                         return old.filectx(path)
                     except KeyError:
-                        raise IOError
+                        return None
 
                 user = opts.get('user') or old.user()
                 date = opts.get('date') or old.date()