diff --git a/requests/__init__.py b/requests/__init__.py
index d638c667..0acbecbf 100644
--- a/requests/__init__.py
+++ b/requests/__init__.py
@@ -44,7 +44,7 @@ import warnings
 from .exceptions import RequestsDependencyWarning
 
 
-def check_compatibility(urllib3_version, chardet_version):
+def check_compatibility(urllib3_version: str, chardet_version: str) -> None:
     urllib3_version = urllib3_version.split('.')
     assert urllib3_version != ['dev']  # Verify urllib3 isn't installed from git.
     # Sometimes, urllib3 only reports its version as 16.1.
@@ -66,7 +66,7 @@ def check_compatibility(urllib3_version, chardet_version):
     assert patch >= 2
 
 
-def _check_cryptography(cryptography_version):
+def _check_cryptography(cryptography_version: str) -> None:
     # cryptography < 1.3.4
     try:
         cryptography_version = list(map(int, cryptography_version.split('.')))