diff -r 61463ff7dc68 Lib/dbm/dumb.py
--- a/Lib/dbm/dumb.py	Wed Oct 23 22:03:45 2013 +0200
+++ b/Lib/dbm/dumb.py	Fri Oct 25 09:39:35 2013 +0300
@@ -118,7 +118,12 @@
 
     sync = _commit
 
+    def _check_closed(self):
+        if self._index is None:
+            raise error('DBM object has already been closed')
+
     def __getitem__(self, key):
+        self._check_closed()
         if isinstance(key, str):
             key = key.encode('utf-8')
         pos, siz = self._index[key]     # may raise KeyError
@@ -165,6 +170,7 @@
         f.close()
 
     def __setitem__(self, key, val):
+        self._check_closed()
         if isinstance(key, str):
             key = key.encode('utf-8')
         elif not isinstance(key, (bytes, bytearray)):
@@ -198,6 +204,7 @@
             # (so that _commit() never gets called).
 
     def __delitem__(self, key):
+        self._check_closed()
         if isinstance(key, str):
             key = key.encode('utf-8')
         # The blocks used by the associated value are lost.
@@ -209,21 +216,26 @@
         self._commit()
 
     def keys(self):
+        self._check_closed()
         return list(self._index.keys())
 
     def items(self):
+        self._check_closed()
         return [(key, self[key]) for key in self._index.keys()]
 
     def __contains__(self, key):
+        self._check_closed()
         if isinstance(key, str):
             key = key.encode('utf-8')
         return key in self._index
 
     def iterkeys(self):
+        self._check_closed()
         return iter(self._index.keys())
     __iter__ = iterkeys
 
     def __len__(self):
+        self._check_closed()
         return len(self._index)
 
     def close(self):
diff -r 61463ff7dc68 Lib/test/test_dbm_dumb.py
--- a/Lib/test/test_dbm_dumb.py	Wed Oct 23 22:03:45 2013 +0200
+++ b/Lib/test/test_dbm_dumb.py	Fri Oct 25 09:39:35 2013 +0300
@@ -8,6 +8,8 @@
 import unittest
 import dbm.dumb as dumbdbm
 from test import support
+import operator
+from functools import partial
 
 _fname = support.TESTFN
 
@@ -186,6 +188,23 @@
             self.assertEqual(expected, got)
             f.close()
 
+    def test_check_closed(self):
+        f = dumbdbm.open(_fname, 'c')
+        f.close()
+
+        for meth in (partial(operator.delitem, f),
+                     partial(operator.setitem, f, 'b'),
+                     partial(operator.getitem, f),
+                     partial(operator.contains, f),
+                     operator.methodcaller('keys'),
+                     operator.methodcaller('iterkeys'),
+                     operator.methodcaller('items'),
+                     len):
+            with self.assertRaises(dumbdbm.error) as cm:
+                meth(f)
+            self.assertEqual(str(cm.exception), 
+                            "DBM object has already been closed")
+
     def tearDown(self):
         _delete_files()