diff --git a/kovot/bot.py b/kovot/bot.py
index 0abcbcc..09d9eef 100644
--- a/kovot/bot.py
+++ b/kovot/bot.py
@@ -3,7 +3,9 @@
 
 
 import logging
+from typing import Optional
 from kovot.mod import ModManager
+from kovot.message import Message
 
 
 # Kovot
@@ -42,7 +44,7 @@ def __init__(self,
         # debug
         self.module_manager.show_mods()
 
-    def talk(self, message):
+    def talk(self, message) -> Optional[Message]:
         message = self.preprocessor.transform(message)
 
         # get responses from mods
@@ -55,6 +57,10 @@ def talk(self, message):
         postprocessed_reponses = [self.postprocessor.transform(res)
                                   for res in selected_resposes]
 
+        if len(postprocessed_reponses) == 0:
+            logging.warn("### no answer candidate ###")
+            return None
+
         # log
         logging.info("### answer candidates ###")
         for response in postprocessed_reponses:
@@ -72,4 +78,5 @@ def run(self, stream):
 
         for message in stream:
             res = self.talk(message)
-            stream.post(res)
+            if res is not None:
+                stream.post(res)
diff --git a/test/test_bot.py b/test/test_bot.py
index e547b7b..083a9fd 100644
--- a/test/test_bot.py
+++ b/test/test_bot.py
@@ -3,6 +3,7 @@
 
 
 import unittest
+from typing import Callable, Iterator, Optional
 from kovot import Bot
 from kovot import Response
 from kovot import Message
@@ -16,6 +17,26 @@ def generate_responses(self, bot, message):
         return [res]
 
 
+class SilentMod:
+    def generate_responses(self, bot, message):
+        return []
+
+
+class StubStream(object):
+    def __init__(self, callback: Optional[Callable[[Response], bool]] = None):
+        self.callback = callback
+        self.callback_num = 0
+
+    def __iter__(self) -> Iterator[Message]:
+        return iter([Message(text="テスト", speaker=Speaker(name="話し✋"))])
+
+    def post(self, response: Response) -> bool:
+        if self.callback:
+            self.callback_num += 1
+            return self.callback(response)
+        return True
+
+
 class BotTest(unittest.TestCase):
     def test_talk(self):
         msg = Message(text="テスト",
@@ -24,3 +45,46 @@ def test_talk(self):
 
         res = bot.talk(msg)
         self.assertEqual(res, Response(score=1.0, text="テスト"))
+
+    def test_talk_nocandidate(self):
+        msg = Message(text="テスト",
+                      speaker=Speaker(name="話し✋"))
+        bot = Bot(mods=[SilentMod()])
+
+        res = bot.talk(msg)
+        self.assertIsNone(res)
+
+    def test_talk_multimod(self):
+        msg = Message(text="テスト",
+                      speaker=Speaker(name="話し✋"))
+        bot = Bot(mods=[EchoMod(), SilentMod()])
+
+        res = bot.talk(msg)
+        self.assertEqual(res, Response(score=1.0, text="テスト"))
+
+    def test_run(self):
+        def cb(res):
+            self.assertEqual(res, Response(score=1.0, text="テスト"))
+            return True
+        bot = Bot(mods=[EchoMod()])
+        stream = StubStream(callback=cb)
+        bot.run(stream=stream)
+        self.assertEqual(stream.callback_num, 1)
+
+    def test_run_nocandidate(self):
+        def cb(res):
+            self.fail()
+            return True
+        bot = Bot(mods=[SilentMod()])
+        stream = StubStream(callback=cb)
+        bot.run(stream=stream)
+        self.assertEqual(stream.callback_num, 0)
+
+    def test_run_multimod(self):
+        def cb(res):
+            self.assertEqual(res, Response(score=1.0, text="テスト"))
+            return True
+        bot = Bot(mods=[EchoMod(), SilentMod()])
+        stream = StubStream(callback=cb)
+        bot.run(stream=stream)
+        self.assertEqual(stream.callback_num, 1)