diff --git a/dev_requirements.txt b/dev_requirements.txt
index 0d91a9e..1891f81 100644
--- a/dev_requirements.txt
+++ b/dev_requirements.txt
@@ -19,6 +19,7 @@ six
 
 # those are the additional packages required to run the tests
 pyfakefs
+certifi
 ddt
 mock
 pytest # optional (python setup.py test works without it), but possible nonetheless
diff --git a/setup.py b/setup.py
index ee088a2..90757f5 100644
--- a/setup.py
+++ b/setup.py
@@ -60,7 +60,7 @@ setup(
     ],
 
     test_suite='tests',
-    tests_require=['pyfakefs>=3.4', 'mock', 'ddt'],
+    tests_require=['pyfakefs>=3.4', 'mock', 'ddt', 'certifi'],
 
     # in order to avoid 'zipimport.ZipImportError: bad local file header'
     zip_safe=False,