diff --git a/apns2/credentials.py b/apns2/credentials.py
index b1eca8b..ed1da1a 100644
--- a/apns2/credentials.py
+++ b/apns2/credentials.py
@@ -1,5 +1,5 @@
 import time
-from typing import Optional, Tuple, TYPE_CHECKING
+from typing import Optional, Tuple, TYPE_CHECKING, Union
 
 import jwt
 
@@ -32,7 +32,7 @@ def get_authorization_header(self, topic: Optional[str]) -> Optional[str]:
 
 # Credentials subclass for certificate authentication
 class CertificateCredentials(Credentials):
-    def __init__(self, cert_file: Optional[str] = None, password: Optional[str] = None,
+    def __init__(self, cert_file: Union[Optional[str], Optional[Tuple[str, str]]] = None, password: Optional[str] = None,
                  cert_chain: Optional[str] = None) -> None:
         ssl_context = init_context(cert=cert_file, cert_password=password)
         if cert_chain: