diff --git a/mopidy/mpd/handler.py b/mopidy/mpd/handler.py
index 475a6bb0..aacad28c 100644
--- a/mopidy/mpd/handler.py
+++ b/mopidy/mpd/handler.py
@@ -70,6 +70,15 @@ class MpdHandler(object):
             response.append(u'OK')
         return response
 
+    @register(r'^ack$')
+    def _ack(self):
+        """
+        Always returns an 'ACK' and not 'OK'.
+
+        Not a part of the MPD protocol.
+        """
+        raise MpdNotImplemented
+
     @register(r'^add "(?P<uri>[^"]*)"$')
     def _add(self, uri):
         raise MpdNotImplemented # TODO
diff --git a/tests/mpd/handlertest.py b/tests/mpd/handlertest.py
index 3f5e4bb5..339d73df 100644
--- a/tests/mpd/handlertest.py
+++ b/tests/mpd/handlertest.py
@@ -66,6 +66,12 @@ class CommandListsTest(unittest.TestCase):
         self.assert_(u'OK' in result)
         self.assertEquals(False, self.h.command_list)
 
+    def test_command_list_with_error(self):
+        self.h.handle_request(u'command_list_begin')
+        self.h.handle_request(u'ack')
+        result = self.h.handle_request(u'command_list_end')
+        self.assert_(u'ACK' in result[-1])
+
     def test_command_list_ok_begin(self):
         result = self.h.handle_request(u'command_list_ok_begin')
         self.assert_(result is None)