diff --git a/Lib/UserString.py b/Lib/UserString.py
index e8e0fed..3251139 100755
--- a/Lib/UserString.py
+++ b/Lib/UserString.py
@@ -146,9 +146,13 @@
     def __hash__(self):
         raise TypeError, "unhashable type (it is mutable)"
     def __setitem__(self, index, sub):
+        if index < 0:
+           index += len(self.data)
         if index < 0 or index >= len(self.data): raise IndexError
         self.data = self.data[:index] + sub + self.data[index+1:]
     def __delitem__(self, index):
+        if index < 0:
+           index += len(self.data)
         if index < 0 or index >= len(self.data): raise IndexError
         self.data = self.data[:index] + self.data[index+1:]
     def __setslice__(self, start, end, sub):
diff --git a/Lib/test/test_userstring.py b/Lib/test/test_userstring.py
index 95f1351..53114db 100755
--- a/Lib/test/test_userstring.py
+++ b/Lib/test/test_userstring.py
@@ -52,20 +52,20 @@
 
     def test_setitem(self):
         s = self.type2test("foo")
-        self.assertRaises(IndexError, s.__setitem__, -1, "bar")
+        self.assertRaises(IndexError, s.__setitem__, -4, "bar")
         self.assertRaises(IndexError, s.__setitem__, 3, "bar")
+        s[-1] = "bar"
+        self.assertEqual(s, "fobar")
         s[0] = "bar"
-        self.assertEqual(s, "baroo")
-        s[4] = "foo"
-        self.assertEqual(s, "barofoo")
+        self.assertEqual(s, "barobar")
 
     def test_delitem(self):
         s = self.type2test("foo")
-        self.assertRaises(IndexError, s.__delitem__, -1)
+        self.assertRaises(IndexError, s.__delitem__, -4)
         self.assertRaises(IndexError, s.__delitem__, 3)
+        del s[-1]
+        self.assertEqual(s, "fo")
         del s[0]
-        self.assertEqual(s, "oo")
-        del s[1]
         self.assertEqual(s, "o")
         del s[0]
         self.assertEqual(s, "")