diff --git a/Lib/test/test_dbm_ndbm.py b/Lib/test/test_dbm_ndbm.py
--- a/Lib/test/test_dbm_ndbm.py
+++ b/Lib/test/test_dbm_ndbm.py
@@ -24,6 +24,7 @@
         self.d[b'bytes'] = b'data'
         self.d['12345678910'] = '019237410982340912840198242'
         self.d.keys()
+        self.assertIn('a', self.d)
         self.assertIn(b'a', self.d)
         self.assertEqual(self.d[b'bytes'], b'data')
         self.d.close()
diff --git a/Modules/_dbmmodule.c b/Modules/_dbmmodule.c
--- a/Modules/_dbmmodule.c
+++ b/Modules/_dbmmodule.c
@@ -215,22 +215,22 @@
                         "DBM object has already been closed");
          return -1;
     }
-    if (PyUnicode_Check(arg)) {
+    if (PyBytes_Check(arg)) {
+        key.dptr = PyBytes_AS_STRING(arg);
+        key.dsize = PyBytes_GET_SIZE(arg);
+    }
+    else if (PyUnicode_Check(arg)) {
         key.dptr = PyUnicode_AsUTF8AndSize(arg, &size);
         key.dsize = size;
         if (key.dptr == NULL)
             return -1;
     }
-    if (!PyBytes_Check(arg)) {
+    else {
         PyErr_Format(PyExc_TypeError,
-                     "dbm key must be string, not %.100s",
+                     "dbm key must be bytes or string, not %.100s",
                      arg->ob_type->tp_name);
         return -1;
     }
-    else {
-        key.dptr = PyBytes_AS_STRING(arg);
-        key.dsize = PyBytes_GET_SIZE(arg);
-    }
     val = dbm_fetch(dp->di_dbm, key);
     return val.dptr != NULL;
 }