diff --git a/requests/models.py b/requests/models.py
index 0ef30251..af364102 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -111,7 +111,7 @@ class Request(object):
         # Dictionary mapping protocol to the URL of the proxy (e.g. {'http': 'foo.bar:3128'})
         self.proxies = dict(proxies or [])
 
-        for proxy_type,uri_ref in self.proxies.items():
+        for proxy_type,uri_ref in list(self.proxies.items()):
             if not uri_ref:
                 del self.proxies[proxy_type]