diff --git a/zerver/lib/bot_lib.py b/zerver/lib/bot_lib.py
index b25b96f521..413b449666 100644
--- a/zerver/lib/bot_lib.py
+++ b/zerver/lib/bot_lib.py
@@ -1,7 +1,7 @@
 import importlib
 import json
 import os
-from typing import Any, Dict
+from typing import Any, Callable, Dict
 
 from django.utils.translation import ugettext as _
 
@@ -45,13 +45,13 @@ class StateHandler:
 
     def __init__(self, user_profile: UserProfile) -> None:
         self.user_profile = user_profile
-        self.marshal = lambda obj: json.dumps(obj)
-        self.demarshal = lambda obj: json.loads(obj)
+        self.marshal: Callable[[object], str] = lambda obj: json.dumps(obj)
+        self.demarshal: Callable[[str], object] = lambda obj: json.loads(obj)
 
-    def get(self, key: str) -> str:
+    def get(self, key: str) -> object:
         return self.demarshal(get_bot_storage(self.user_profile, key))
 
-    def put(self, key: str, value: str) -> None:
+    def put(self, key: str, value: object) -> None:
         set_bot_storage(self.user_profile, [(key, self.marshal(value))])
 
     def remove(self, key: str) -> None:
diff --git a/zerver/lib/bot_storage.py b/zerver/lib/bot_storage.py
index 1b43644f0b..db6d1d7474 100644
--- a/zerver/lib/bot_storage.py
+++ b/zerver/lib/bot_storage.py
@@ -32,10 +32,8 @@ def set_bot_storage(bot_profile: UserProfile, entries: List[Tuple[str, str]]) ->
     storage_size_limit = settings.USER_STATE_SIZE_LIMIT
     storage_size_difference = 0
     for key, value in entries:
-        if not isinstance(key, str):
-            raise StateError(f"Key type is {type(key)}, but should be str.")
-        if not isinstance(value, str):
-            raise StateError(f"Value type is {type(value)}, but should be str.")
+        assert isinstance(key, str), "Key type should be str."
+        assert isinstance(value, str), "Value type should be str."
         storage_size_difference += (len(key) + len(value)) - get_bot_storage_size(bot_profile, key)
     new_storage_size = get_bot_storage_size(bot_profile) + storage_size_difference
     if new_storage_size > storage_size_limit:
diff --git a/zerver/tests/test_service_bot_system.py b/zerver/tests/test_service_bot_system.py
index eba9854448..1a2aec0a23 100644
--- a/zerver/tests/test_service_bot_system.py
+++ b/zerver/tests/test_service_bot_system.py
@@ -10,6 +10,7 @@ from zerver.lib.bot_config import ConfigError, load_bot_config_template, set_bot
 from zerver.lib.bot_lib import EmbeddedBotEmptyRecipientsList, EmbeddedBotHandler, StateHandler
 from zerver.lib.bot_storage import StateError
 from zerver.lib.test_classes import ZulipTestCase
+from zerver.lib.validator import check_string
 from zerver.models import Recipient, UserProfile, get_realm
 
 BOT_TYPE_TO_QUEUE_NAME = {
@@ -198,19 +199,9 @@ class TestServiceBotStateHandler(ZulipTestCase):
     def test_marshaling(self) -> None:
         storage = StateHandler(self.bot_profile)
         serializable_obj = {'foo': 'bar', 'baz': [42, 'cux']}
-        storage.put('some key', serializable_obj)  # type: ignore[arg-type] # Ignore for testing.
+        storage.put('some key', serializable_obj)
         self.assertEqual(storage.get('some key'), serializable_obj)
 
-    def test_invalid_calls(self) -> None:
-        storage = StateHandler(self.bot_profile)
-        storage.marshal = lambda obj: obj
-        storage.demarshal = lambda obj: obj
-        serializable_obj = {'foo': 'bar', 'baz': [42, 'cux']}
-        with self.assertRaisesMessage(StateError, "Value type is <class 'dict'>, but should be str."):
-            storage.put('some key', serializable_obj)  # type: ignore[arg-type] # We intend to test an invalid type.
-        with self.assertRaisesMessage(StateError, "Key type is <class 'dict'>, but should be str."):
-            storage.put(serializable_obj, 'some value')  # type: ignore[arg-type] # We intend to test an invalid type.
-
     # Reduce maximal storage size for faster test string construction.
     @override_settings(USER_STATE_SIZE_LIMIT=100)
     def test_storage_limit(self) -> None:
@@ -218,7 +209,7 @@ class TestServiceBotStateHandler(ZulipTestCase):
 
         # Disable marshaling for storing a string whose size is
         # equivalent to the size of the stored object.
-        storage.marshal = lambda obj: obj
+        storage.marshal = lambda obj: check_string("obj", obj)
         storage.demarshal = lambda obj: obj
 
         key = 'capacity-filling entry'
@@ -283,10 +274,10 @@ class TestServiceBotStateHandler(ZulipTestCase):
         self.assertEqual(result.json()['storage'], updated_dict)
 
         # Assert errors on invalid requests.
-        params = {
-            'keys': ["This is a list, but should be a serialized string."],  # type: ignore[dict-item] # Ignore 'incompatible type "str": "List[str]"; expected "str": "str"' for testing
+        invalid_params = {
+            'keys': ["This is a list, but should be a serialized string."],
         }
-        result = self.client_get('/json/bot_storage', params)
+        result = self.client_get('/json/bot_storage', invalid_params)
         self.assert_json_error(result, 'Argument "keys" is not valid JSON.')
 
         params = {