diff --git a/requests/utils.py b/requests/utils.py
index 6feaaf3832..1ac81fcedf 100644
--- a/requests/utils.py
+++ b/requests/utils.py
@@ -684,7 +684,7 @@ def should_bypass_proxies(url, no_proxy):
     return False
 
 
-def get_environ_proxies(url, no_proxy):
+def get_environ_proxies(url, no_proxy=None):
     """
     Return a dict of environment proxies.