diff --git a/hgext/bookmarks.py b/hgext/bookmarks.py
--- a/hgext/bookmarks.py
+++ b/hgext/bookmarks.py
@@ -44,8 +44,14 @@ def write(repo):
     can be copied back on rollback.
     '''
     refs = repo._bookmarks
-    if os.path.exists(repo.join('bookmarks')):
-        util.copyfile(repo.join('bookmarks'), repo.join('undo.bookmarks'))
+
+    try:
+        bms = repo.opener('bookmarks').read()
+    except IOError:
+        bms = None
+    if bms is not None:
+        repo.opener('undo.bookmarks', 'w').write(bms)
+
     if repo._bookmarkcurrent not in refs:
         setcurrent(repo, None)
     wlock = repo.wlock()