diff --git a/requests/adapters.py b/requests/adapters.py
index 383ea9ab..44e0786c 100644
--- a/requests/adapters.py
+++ b/requests/adapters.py
@@ -13,7 +13,7 @@ import socket
 from .models import Response
 from .packages.urllib3.poolmanager import PoolManager, proxy_from_url
 from .hooks import dispatch_hook
-from .compat import urlparse
+from .compat import urlparse, basestring
 from .utils import DEFAULT_CA_BUNDLE_PATH, get_encoding_from_headers
 from .structures import CaseInsensitiveDict
 from .packages.urllib3.exceptions import MaxRetryError