diff --git a/Lib/dbm/__init__.py b/Lib/dbm/__init__.py
index 9fdd414..2082e07 100644
--- a/Lib/dbm/__init__.py
+++ b/Lib/dbm/__init__.py
@@ -48,27 +48,26 @@
     pass
 
 _names = ['dbm.bsd', 'dbm.gnu', 'dbm.ndbm', 'dbm.dumb']
-_errors = [error]
 _defaultmod = None
 _modules = {}
 
-for _name in _names:
-    try:
-        _mod = __import__(_name, fromlist=['open'])
-    except ImportError:
-        continue
-    if not _defaultmod:
-        _defaultmod = _mod
-    _modules[_name] = _mod
-    _errors.append(_mod.error)
-
-if not _defaultmod:
-    raise ImportError("no dbm clone found; tried %s" % _names)
-
-error = tuple(_errors)
+error = (error, IOError)
 
 
 def open(file, flag = 'r', mode = 0o666):
+    global _defaultmod
+    if _defaultmod is None:
+        for name in _names:
+            try:
+                mod = __import__(name, fromlist=['open'])
+            except ImportError:
+                continue
+            if not _defaultmod:
+                _defaultmod = mod
+            _modules[name] = mod
+        if not _defaultmod:
+            raise ImportError("no dbm clone found; tried %s" % _names)
+
     # guess the type of an existing database
     result = whichdb(file)
     if result is None:
@@ -81,19 +80,14 @@
     elif result == "":
         # db type cannot be determined
         raise error("db type could not be determined")
+    elif result not in _modules:
+        raise error("db type is {0}, but the module is not "
+                    "available".format(result))
     else:
         mod = _modules[result]
     return mod.open(file, flag, mode)
 
 
-try:
-    from dbm import ndbm
-    _dbmerror = ndbm.error
-except ImportError:
-    ndbm = None
-    # just some sort of valid exception which might be raised in the ndbm test
-    _dbmerror = IOError
-
 def whichdb(filename):
     """Guess which db package to use to open a db file.
 
@@ -129,7 +123,7 @@
                 d = ndbm.open(filename)
                 d.close()
                 return "dbm.ndbm"
-        except (IOError, _dbmerror):
+        except IOError:
             pass
 
     # Check for dumbdbm next -- this has a .dir and a .dat file
diff --git a/Lib/dbm/bsd.py b/Lib/dbm/bsd.py
index 8353f50..2dccadb 100644
--- a/Lib/dbm/bsd.py
+++ b/Lib/dbm/bsd.py
@@ -4,7 +4,8 @@
 
 __all__ = ["error", "open"]
 
-error = bsddb.error
+class error(bsddb.error, IOError):
+    pass
 
 def open(file, flag = 'r', mode=0o666):
     return bsddb.hashopen(file, flag, mode)
diff --git a/Lib/test/test_dbm.py b/Lib/test/test_dbm.py
index aab1388..41c37cb 100644
--- a/Lib/test/test_dbm.py
+++ b/Lib/test/test_dbm.py
@@ -14,11 +14,13 @@
 # setting dbm to use each in turn, and yielding that module
 #
 def dbm_iterator():
-    old_default = dbm._defaultmod
-    for module in dbm._modules.values():
-        dbm._defaultmod = module
-        yield module
-    dbm._defaultmod = old_default
+    for name in dbm._names:
+        try:
+            mod = __import__(name, fromlist=['open'])
+        except ImportError:
+            continue
+        dbm._modules[name] = mod
+        yield mod
 
 #
 # Clean up all scratch databases we might have created during testing
@@ -40,8 +42,20 @@
              'g': b'intended',
              }
 
-    def __init__(self, *args):
-        unittest.TestCase.__init__(self, *args)
+    def init_db(self):
+        f = dbm.open(_fname, 'n')
+        for k in self._dict:
+            f[k.encode("ascii")] = self._dict[k]
+        f.close()
+
+    def keys_helper(self, f):
+        keys = sorted(k.decode("ascii") for k in f.keys())
+        dkeys = sorted(self._dict.keys())
+        self.assertEqual(keys, dkeys)
+        return keys
+
+    def test_error(self):
+        self.assert_(issubclass(self.module.error, IOError))
 
     def test_anydbm_creation(self):
         f = dbm.open(_fname, 'c')
@@ -83,22 +97,11 @@
         for key in self._dict:
             self.assertEqual(self._dict[key], f[key.encode("ascii")])
 
-    def init_db(self):
-        f = dbm.open(_fname, 'n')
-        for k in self._dict:
-            f[k.encode("ascii")] = self._dict[k]
-        f.close()
-
-    def keys_helper(self, f):
-        keys = sorted(k.decode("ascii") for k in f.keys())
-        dkeys = sorted(self._dict.keys())
-        self.assertEqual(keys, dkeys)
-        return keys
-
     def tearDown(self):
         delete_files()
 
     def setUp(self):
+        dbm._defaultmod = self.module
         delete_files()
 
 
@@ -137,11 +140,11 @@
 
 
 def test_main():
-    try:
-        for module in dbm_iterator():
-            test.support.run_unittest(AnyDBMTestCase, WhichDBTestCase)
-    finally:
-        delete_files()
+    classes = [WhichDBTestCase]
+    for mod in dbm_iterator():
+        classes.append(type("TestCase-" + mod.__name__, (AnyDBMTestCase,),
+                            {'module': mod}))
+    test.support.run_unittest(*classes)
 
 if __name__ == "__main__":
     test_main()
diff --git a/Modules/_dbmmodule.c b/Modules/_dbmmodule.c
index ddfd4cd..7e80381 100644
--- a/Modules/_dbmmodule.c
+++ b/Modules/_dbmmodule.c
@@ -401,7 +401,8 @@
 		return;
 	d = PyModule_GetDict(m);
 	if (DbmError == NULL)
-		DbmError = PyErr_NewException("_dbm.error", NULL, NULL);
+		DbmError = PyErr_NewException("_dbm.error",
+					      PyExc_IOError, NULL);
 	s = PyUnicode_FromString(which_dbm);
 	if (s != NULL) {
 		PyDict_SetItemString(d, "library", s);
diff --git a/Modules/_gdbmmodule.c b/Modules/_gdbmmodule.c
index 6c75819..abc8837 100644
--- a/Modules/_gdbmmodule.c
+++ b/Modules/_gdbmmodule.c
@@ -523,7 +523,7 @@
     if (m == NULL)
 	return;
     d = PyModule_GetDict(m);
-    DbmError = PyErr_NewException("_gdbm.error", NULL, NULL);
+    DbmError = PyErr_NewException("_gdbm.error", PyExc_IOError, NULL);
     if (DbmError != NULL) {
         PyDict_SetItemString(d, "error", DbmError);
         s = PyUnicode_FromString(dbmmodule_open_flags);