diff --git a/Lib/test/test_urllib2.py b/Lib/test/test_urllib2.py
index b6e4e91..8cd5ca0 100644
--- a/Lib/test/test_urllib2.py
+++ b/Lib/test/test_urllib2.py
@@ -5,6 +5,7 @@
 import io
 import socket
 import array
+import sys
 
 import urllib.request
 # The proxy bypass method imported below has logic specific to the OSX
@@ -1162,6 +1163,8 @@
         self.assertEqual(req.get_host(), "proxy.example.com:3128")
         self.assertEqual(req.get_header("Proxy-authorization"),"FooBar")
 
+    # TODO: This should be only for OSX
+    @unittest.skipUnless(sys.platform == 'darwin', "only relevant for OSX")
     def test_osx_proxy_bypass(self):
         bypass = {
             'exclude_simple': False,
@@ -1265,6 +1268,26 @@
         # _test_basic_auth called .open() twice)
         self.assertEqual(opener.recorded, ["digest", "basic"]*2)
 
+    def test_unsupported_auth_digest_handler(self):
+        opener = OpenerDirector()
+        # While using DigestAuthHandler
+        digest_auth_handler = urllib.request.HTTPDigestAuthHandler(None)
+        http_handler = MockHTTPHandler(
+            401, 'WWW-Authenticate: Kerberos\r\n\r\n')
+        opener.add_handler(digest_auth_handler)
+        opener.add_handler(http_handler)
+        self.assertRaises(ValueError,opener.open,"http://www.example.com")
+
+    def test_unsupported_auth_basic_handler(self):
+        # While using BasicAuthHandler
+        opener = OpenerDirector()
+        basic_auth_handler = urllib.request.HTTPBasicAuthHandler(None)
+        http_handler = MockHTTPHandler(
+            401, 'WWW-Authenticate: NTLM\r\n\r\n')
+        opener.add_handler(basic_auth_handler)
+        opener.add_handler(http_handler)
+        self.assertRaises(ValueError,opener.open,"http://www.example.com")
+
     def _test_basic_auth(self, opener, auth_handler, auth_header,
                          realm, http_handler, password_manager,
                          request_url, protected_url):
@@ -1302,6 +1325,7 @@
         self.assertEqual(len(http_handler.requests), 1)
         self.assertFalse(http_handler.requests[0].has_header(auth_header))
 
+
 class MiscTests(unittest.TestCase):
 
     def test_build_opener(self):
diff --git a/Lib/urllib/request.py b/Lib/urllib/request.py
index c80b7d1..e98a976 100644
--- a/Lib/urllib/request.py
+++ b/Lib/urllib/request.py
@@ -825,14 +825,20 @@
             self.retried += 1
 
         if authreq:
-            mo = AbstractBasicAuthHandler.rx.search(authreq)
-            if mo:
-                scheme, quote, realm = mo.groups()
-                if scheme.lower() == 'basic':
-                    response = self.retry_http_basic_auth(host, req, realm)
-                    if response and response.code != 401:
-                        self.retried = 0
-                    return response
+            scheme = authreq.split()[0]
+            if not scheme.lower() == 'basic':
+                raise ValueError("AbstractBasicAuthHandler does not"
+                                 " support the following scheme: '%s'" %
+                                 scheme)
+            else:
+                mo = AbstractBasicAuthHandler.rx.search(authreq)
+                if mo:
+                    scheme, quote, realm = mo.groups()
+                    if scheme.lower() == 'basic':
+                        response = self.retry_http_basic_auth(host, req, realm)
+                        if response and response.code != 401:
+                            self.retried = 0
+                        return response
 
     def retry_http_basic_auth(self, host, req, realm):
         user, pw = self.passwd.find_user_password(realm, host)
@@ -918,6 +924,9 @@
             scheme = authreq.split()[0]
             if scheme.lower() == 'digest':
                 return self.retry_http_digest_auth(req, authreq)
+            elif not scheme.lower() == 'basic':
+                raise ValueError("AbstractDigestAuthHandler does not support"
+                                 " the following scheme: '%s'" % scheme)
 
     def retry_http_digest_auth(self, req, auth):
         token, challenge = auth.split(' ', 1)