diff --git a/.github/workflows/ci-cd.yml b/.github/workflows/ci-cd.yml
index 76b4b5d..4735108 100644
--- a/.github/workflows/ci-cd.yml
+++ b/.github/workflows/ci-cd.yml
@@ -50,7 +50,7 @@ jobs:
         pip install -r test-requirements.txt -r requirements.txt
     - name: Run tests
       run: |
-        mypy --check tinify
+        mypy tinify
 
   Integration_tests:
     if: github.event_name == 'push'
diff --git a/tinify/client.py b/tinify/client.py
index a170b05..ae27548 100644
--- a/tinify/client.py
+++ b/tinify/client.py
@@ -5,6 +5,7 @@
 import os
 import platform
 import requests
+from requests.structures import CaseInsensitiveDict
 import requests.exceptions
 from requests.compat import json # type: ignore
 import traceback
@@ -31,9 +32,9 @@ def __init__(self, key, app_identifier=None, proxy=None):  # type: (str, Optiona
         if proxy:
             self.session.proxies = {'https': proxy}
         self.session.auth = ('api', key)
-        self.session.headers = {
+        self.session.headers = CaseInsensitiveDict({
             'user-agent': self.USER_AGENT + ' ' + app_identifier if app_identifier else self.USER_AGENT,
-        }
+        })
         self.session.verify = os.path.join(os.path.dirname(os.path.realpath(__file__)), 'data', 'cacert.pem')
 
     def __enter__(self):  # type: () -> Client