Index: Lib/test/test_posixpath.py
===================================================================
--- Lib/test/test_posixpath.py	(revision 70217)
+++ Lib/test/test_posixpath.py	(working copy)
@@ -345,6 +345,11 @@
             self.assert_(isinstance(posixpath.expanduser("~root/"), basestring))
             self.assert_(isinstance(posixpath.expanduser("~foo/"), basestring))
 
+            orig_home = os.environ['HOME']
+            os.environ['HOME'] = '/'
+            self.assertEqual(posixpath.expanduser("~"), "/")
+            os.environ['HOME'] = orig_home
+
         self.assertRaises(TypeError, posixpath.expanduser)
 
     def test_expandvars(self):
Index: Lib/posixpath.py
===================================================================
--- Lib/posixpath.py	(revision 70217)
+++ Lib/posixpath.py	(working copy)
@@ -262,7 +262,7 @@
         except KeyError:
             return path
         userhome = pwent.pw_dir
-    userhome = userhome.rstrip('/')
+    userhome = userhome.rstrip('/') or userhome
     return userhome + path[i:]