diff --git a/requests/adapters.py b/requests/adapters.py
index 206978b3..96d64327 100644
--- a/requests/adapters.py
+++ b/requests/adapters.py
@@ -275,7 +275,8 @@ class HTTPAdapter(BaseAdapter):
         :param proxies: A dictionary of schemes or schemes and hosts to proxy URLs.
         """
         proxy = select_proxy(request.url, proxies)
-        if proxy and not request.url.lower().startswith('https'):
+        scheme = urlparse(request.url.lower()).scheme
+        if proxy and scheme != 'https':
             url = urldefragauth(request.url)
         else:
             url = request.path_url