diff --git a/Lib/ipaddress.py b/Lib/ipaddress.py
index ca3031a..f4d815b 100644
--- a/Lib/ipaddress.py
+++ b/Lib/ipaddress.py
@@ -636,12 +636,12 @@ class _BaseNetwork(_IPAddressBase):
         broadcast = int(self.broadcast_address)
         if n >= 0:
             if network + n > broadcast:
-                raise IndexError
+                raise IndexError("address out of range")
             return self._address_class(network + n)
         else:
             n += 1
             if broadcast + n < network:
-                raise IndexError
+                raise IndexError("address out of range")
             return self._address_class(broadcast + n)
 
     def __lt__(self, other):
diff --git a/Lib/test/test_ipaddress.py b/Lib/test/test_ipaddress.py
index 3edea08..d344720 100644
--- a/Lib/test/test_ipaddress.py
+++ b/Lib/test/test_ipaddress.py
@@ -1176,6 +1176,7 @@ class IpaddrUnitTest(unittest.TestCase):
 
         self.assertEqual(str(self.ipv6_network[5]),
                          '2001:658:22a:cafe::5')
+        self.assertRaises(IndexError, self.ipv6_network.__getitem__, 1<<64)
 
     def testGetitem(self):
         # http://code.google.com/p/ipaddr-py/issues/detail?id=15