diff --git a/mypy.ini b/mypy.ini
index 9a79176..d980ad0 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -2,6 +2,7 @@
 check_untyped_defs = True
 disallow_any_generics = True
 strict_optional = True
+no_implicit_optional = True
 
 incremental = True
 scripts_are_modules = True
diff --git a/zulip/zulip/__init__.py b/zulip/zulip/__init__.py
index 7efc14a..d7c5032 100644
--- a/zulip/zulip/__init__.py
+++ b/zulip/zulip/__init__.py
@@ -286,7 +286,7 @@ class Client(object):
                  site=None, client=None,
                  cert_bundle=None, insecure=None,
                  client_cert=None, client_cert_key=None):
-        # type: (Optional[str], Optional[str], Optional[str], bool, bool, Optional[str], Optional[str], Optional[str], bool, Optional[str], Optional[str]) -> None
+        # type: (Optional[str], Optional[str], Optional[str], bool, bool, Optional[str], Optional[str], Optional[str], Optional[bool], Optional[str], Optional[str]) -> None
         if client is None:
             client = _default_client()
 
diff --git a/zulip_bots/zulip_bots/bots/idonethis/idonethis.py b/zulip_bots/zulip_bots/bots/idonethis/idonethis.py
index be289bc..88af20e 100644
--- a/zulip_bots/zulip_bots/bots/idonethis/idonethis.py
+++ b/zulip_bots/zulip_bots/bots/idonethis/idonethis.py
@@ -3,7 +3,7 @@ import requests
 import logging
 import re
 
-from typing import Any, Dict
+from typing import Any, Dict, Optional
 
 API_BASE_URL = "https://beta.idonethis.com/api/v2"
 
@@ -25,7 +25,7 @@ class UnknownCommandSyntax(Exception):
 class UnspecifiedProblemException(Exception):
     pass
 
-def make_API_request(endpoint: str, method: str="GET", body: Dict[str, str]=None) -> Any:
+def make_API_request(endpoint: str, method: str="GET", body: Optional[Dict[str, str]]=None) -> Any:
     headers = {'Authorization': 'Token ' + api_key}
     if method == "GET":
         r = requests.get(API_BASE_URL + endpoint, headers=headers)
@@ -52,7 +52,7 @@ def api_show_team(hash_id: str) -> Any:
 def api_show_users(hash_id: str) -> Any:
     return make_API_request("/teams/{}/members".format(hash_id))
 
-def api_list_entries(team_id: str=None) -> Any:
+def api_list_entries(team_id: Optional[str]=None) -> Any:
     if team_id:
         return make_API_request("/entries?team_id={}".format(team_id))
     else:
diff --git a/zulip_bots/zulip_bots/bots/xkcd/xkcd.py b/zulip_bots/zulip_bots/bots/xkcd/xkcd.py
index e12616e..bf44d20 100644
--- a/zulip_bots/zulip_bots/bots/xkcd/xkcd.py
+++ b/zulip_bots/zulip_bots/bots/xkcd/xkcd.py
@@ -3,7 +3,7 @@ import random
 import logging
 import requests
 
-from typing import Any, Dict
+from typing import Any, Dict, Optional
 
 XKCD_TEMPLATE_URL = 'https://xkcd.com/%s/info.0.json'
 LATEST_XKCD_URL = 'https://xkcd.com/info.0.json'
@@ -84,7 +84,7 @@ def get_xkcd_bot_response(message: Dict[str, str]) -> str:
                                            fetched['alt'],
                                            fetched['img']))
 
-def fetch_xkcd_query(mode: int, comic_id: str = None) -> Dict[str, str]:
+def fetch_xkcd_query(mode: int, comic_id: Optional[str]=None) -> Dict[str, str]:
     try:
         if mode == XkcdBotCommand.LATEST:  # Fetch the latest comic strip.
             url = LATEST_XKCD_URL
diff --git a/zulip_bots/zulip_bots/bots/yoda/test_yoda.py b/zulip_bots/zulip_bots/bots/yoda/test_yoda.py
index fee9931..9596f14 100644
--- a/zulip_bots/zulip_bots/bots/yoda/test_yoda.py
+++ b/zulip_bots/zulip_bots/bots/yoda/test_yoda.py
@@ -1,6 +1,8 @@
 from zulip_bots.bots.yoda.yoda import ServiceUnavailableError
 from zulip_bots.test_lib import BotTestCase
 
+from typing import Optional
+
 class TestYodaBot(BotTestCase):
     bot_name = "yoda"
 
@@ -18,7 +20,7 @@ class TestYodaBot(BotTestCase):
             @mention-bot You will learn how to speak like me someday.
             '''
 
-    def _test(self, message: str, response: str, fixture: str=None) -> None:
+    def _test(self, message: str, response: str, fixture: Optional[str]=None) -> None:
         with self.mock_config_info({'api_key': '12345678'}):
             if fixture is not None:
                 with self.mock_http_conversation(fixture):