diff -r d7a59e6f48df Lib/dbm/__init__.py
--- a/Lib/dbm/__init__.py	Sun Jul 07 09:54:08 2013 +0200
+++ b/Lib/dbm/__init__.py	Sun Jul 07 14:33:33 2013 +0400
@@ -44,6 +44,11 @@
 
 error = (error, OSError)
 
+try:
+    from dbm import ndbm
+except ImportError:
+    ndbm = None
+
 
 def open(file, flag='r', mode=0o666):
     """Open or create database at path given by *file*.
diff -r d7a59e6f48df Lib/test/test_dbm.py
--- a/Lib/test/test_dbm.py	Sun Jul 07 09:54:08 2013 +0200
+++ b/Lib/test/test_dbm.py	Sun Jul 07 14:33:33 2013 +0400
@@ -9,6 +9,11 @@
 # Skip tests if dbm module doesn't exist.
 dbm = test.support.import_module('dbm')
 
+try:
+    from dbm import ndbm
+except ImportError:
+    ndbm = None
+
 _fname = test.support.TESTFN
 
 #
@@ -130,7 +135,7 @@
             delete_files()
             f = module.open(_fname, 'c')
             f.close()
-            self.assertEqual(name, dbm.whichdb(_fname))
+            self.assertEqual(name, self.dbm.whichdb(_fname))
             # Now add a key
             f = module.open(_fname, 'w')
             f[b"1"] = b"1"
@@ -139,7 +144,15 @@
             # and read it
             self.assertTrue(f[b"1"] == b"1")
             f.close()
-            self.assertEqual(name, dbm.whichdb(_fname))
+            self.assertEqual(name, self.dbm.whichdb(_fname))
+
+    @unittest.skipUnless(ndbm, reason='Test requires ndbm')
+    def test_whichdb_ndbm(self):
+        # Issue 17198: check that ndbm which is referenced in whichdb is defined
+        db_file = '{}_ndbm.db'.format(_fname)
+        with open(db_file, 'w'):
+            self.addCleanup(test.support.unlink, db_file)
+        self.assertIsNone(self.dbm.whichdb(db_file[:-3]))
 
     def tearDown(self):
         delete_files()
@@ -149,6 +162,7 @@
         self.filename = test.support.TESTFN
         self.d = dbm.open(self.filename, 'c')
         self.d.close()
+        self.dbm = test.support.import_fresh_module('dbm')
 
     def test_keys(self):
         self.d = dbm.open(self.filename, 'c')