diff --git a/requests/adapters.py b/requests/adapters.py
index 7e65c78e..e6cb50ed 100644
--- a/requests/adapters.py
+++ b/requests/adapters.py
@@ -15,7 +15,7 @@ from .packages.urllib3.poolmanager import PoolManager, ProxyManager
 from .packages.urllib3.response import HTTPResponse
 from .compat import urlparse, basestring, urldefrag, unquote
 from .utils import (DEFAULT_CA_BUNDLE_PATH, get_encoding_from_headers,
-                    prepend_scheme_if_needed, get_auth_from_url)
+                    except_on_missing_scheme, get_auth_from_url)
 from .structures import CaseInsensitiveDict
 from .packages.urllib3.exceptions import MaxRetryError
 from .packages.urllib3.exceptions import TimeoutError
@@ -193,7 +193,7 @@ class HTTPAdapter(BaseAdapter):
         proxy = proxies.get(urlparse(url.lower()).scheme)
 
         if proxy:
-            proxy = prepend_scheme_if_needed(proxy, urlparse(url.lower()).scheme)
+            except_on_missing_scheme(proxy)
             conn = ProxyManager(self.poolmanager.connection_from_url(proxy))
         else:
             conn = self.poolmanager.connection_from_url(url.lower())
diff --git a/requests/utils.py b/requests/utils.py
index e592c8bb..347f2be1 100644
--- a/requests/utils.py
+++ b/requests/utils.py
@@ -25,6 +25,7 @@ from .compat import (quote, urlparse, bytes, str, OrderedDict, urlunparse,
                      is_py2, is_py3, builtin_str, getproxies, proxy_bypass)
 from .cookies import RequestsCookieJar, cookiejar_from_dict
 from .structures import CaseInsensitiveDict
+from .exceptions import MissingSchema
 
 _hush_pyflakes = (RequestsCookieJar,)
 
@@ -524,18 +525,13 @@ def guess_json_utf(data):
     return None
 
 
-def prepend_scheme_if_needed(url, new_scheme):
-    '''Given a URL that may or may not have a scheme, prepend the given scheme.
-    Does not replace a present scheme with the one provided as an argument.'''
-    scheme, netloc, path, params, query, fragment = urlparse(url, new_scheme)
+def except_on_missing_scheme(url):
+    """Given a URL, raise a MissingSchema exception if the scheme is missing.
+    """
+    scheme, netloc, path, params, query, fragment = urlparse(url)
 
-    # urlparse is a finicky beast, and sometimes decides that there isn't a
-    # netloc present. Assume that it's being over-cautious, and switch netloc
-    # and path if urlparse decided there was no netloc.
-    if not netloc:
-        netloc, path = path, netloc
-
-    return urlunparse((scheme, netloc, path, params, query, fragment))
+    if not scheme:
+        raise MissingSchema('Proxy URLs must have explicit schemes.')
 
 
 def get_auth_from_url(url):