diff --git a/tools/run-mypy b/tools/run-mypy
index f5bd5e6..17a6b3a 100755
--- a/tools/run-mypy
+++ b/tools/run-mypy
@@ -46,6 +46,8 @@ force_include = [
     "zulip_bots/zulip_bots/bots/googlesearch/test_googlesearch.py",
     "zulip_bots/zulip_bots/bots/help/help.py",
     "zulip_bots/zulip_bots/bots/help/test_help.py",
+    "zulip_bots/zulip_bots/bots/incrementor/incrementor.py",
+    "zulip_bots/zulip_bots/bots/incrementor/test_incrementor.py",
     "zulip_bots/zulip_bots/bots/virtual_fs/virtual_fs.py",
     "zulip_bots/zulip_bots/bots/virtual_fs/test_virtual_fs.py",
     "zulip_bots/zulip_bots/bots/weather/test_weather.py",
diff --git a/zulip_bots/zulip_bots/bots/incrementor/incrementor.py b/zulip_bots/zulip_bots/bots/incrementor/incrementor.py
index dbdcb3d..6a0994e 100644
--- a/zulip_bots/zulip_bots/bots/incrementor/incrementor.py
+++ b/zulip_bots/zulip_bots/bots/incrementor/incrementor.py
@@ -1,5 +1,6 @@
 # See readme.md for instructions on running this code.
 
+from zulip_bots.lib import ExternalBotHandler
 
 class IncrementorHandler(object):
     META = {
@@ -7,7 +8,7 @@ class IncrementorHandler(object):
         'description': 'Example bot to test the update_message() function.',
     }
 
-    def usage(self):
+    def usage(self) -> str:
         return '''
         This is a boilerplate bot that makes use of the
         update_message function. For the first @-mention, it initially
@@ -15,23 +16,28 @@ class IncrementorHandler(object):
         is @-mentioned, this number will be incremented in the same message.
         '''
 
-    def initialize(self, bot_handler):
+    def initialize(self, bot_handler: ExternalBotHandler) -> None:
         storage = bot_handler.storage
         if not storage.contains('number') or not storage.contains('message_id'):
             storage.put('number', 0)
             storage.put('message_id', None)
 
-    def handle_message(self, message, bot_handler):
+    def handle_message(self, message: dict, bot_handler: ExternalBotHandler) -> None:
         storage = bot_handler.storage
         num = storage.get('number')
-        storage.put('number', num + 1)
+
+        # num should already be an int, but we do `int()` to force an
+        # explicit type check
+        num = int(num) + 1
+
+        storage.put('number', num)
         if storage.get('message_id') is None:
-            result = bot_handler.send_reply(message, str(storage.get('number')))
+            result = bot_handler.send_reply(message, str(num))
             storage.put('message_id', result['id'])
         else:
             bot_handler.update_message(dict(
-                message_id = storage.get('message_id'),
-                content = str(storage.get('number'))
+                message_id=storage.get('message_id'),
+                content=str(num)
             ))
 
 
diff --git a/zulip_bots/zulip_bots/bots/incrementor/test_incrementor.py b/zulip_bots/zulip_bots/bots/incrementor/test_incrementor.py
index 34e85b8..6d34480 100644
--- a/zulip_bots/zulip_bots/bots/incrementor/test_incrementor.py
+++ b/zulip_bots/zulip_bots/bots/incrementor/test_incrementor.py
@@ -10,7 +10,7 @@ from zulip_bots.test_lib import (
 class TestIncrementorBot(StubBotTestCase):
     bot_name = "incrementor"
 
-    def test_bot(self):
+    def test_bot(self) -> None:
         bot = get_bot_message_handler(self.bot_name)
         bot_handler = StubBotHandler()