diff --git a/requests/__init__.py b/requests/__init__.py
index 5c299a8..c016424 100644
--- a/requests/__init__.py
+++ b/requests/__init__.py
@@ -42,9 +42,10 @@
 import chardet
 import warnings
 from .exceptions import RequestsDependencyWarning
+from .import types
 
 
-def check_compatibility(urllib3_version: str, chardet_version: str) -> None:
+def check_compatibility(urllib3_version: types.String, chardet_version: types.String) -> None:
     urllib3_version = urllib3_version.split('.')  # type: ignore
     assert urllib3_version != [
         'dev'
@@ -68,7 +69,7 @@
     assert patch >= 2  # type: ignore
 
 
-def _check_cryptography(cryptography_version: str) -> None:
+def _check_cryptography(cryptography_version: types.String) -> None:
     # cryptography < 1.3.4
     try:
         cryptography_version = list(
diff --git a/requests/types.py b/requests/types.py
index 4b60986..0ea5d7b 100644
--- a/requests/types.py
+++ b/requests/types.py
@@ -68,3 +68,5 @@
 Sequence = List
 Filename = str
 KeyValueList = List[Tuple[Text, Text]]
+String = str
+Boolean = Union[bool, None]