diff --git a/oauth2client/client.py b/oauth2client/client.py
index c5b7214..ae0301d 100644
--- a/oauth2client/client.py
+++ b/oauth2client/client.py
@@ -409,7 +409,9 @@ def clean_headers(headers):
   clean = {}
   try:
     for k, v in six.iteritems(headers):
-      clean[k.encode('ascii')] = v.encode('ascii')
+      clean_k = k if isinstance(k, bytes) else str(k).encode('ascii')
+      clean_v = v if isinstance(v, bytes) else str(v).encode('ascii')
+      clean[clean_k] = clean_v
   except UnicodeEncodeError:
     raise NonAsciiHeaderError(k + ': ' + v)
   return clean
diff --git a/tests/test_oauth2client.py b/tests/test_oauth2client.py
index fea08c8..82ddb92 100644
--- a/tests/test_oauth2client.py
+++ b/tests/test_oauth2client.py
@@ -598,6 +598,37 @@ class BasicCredentialsTests(unittest.TestCase):
     instance = OAuth2Credentials.from_json(json.dumps(data))
     self.assertTrue(isinstance(instance, OAuth2Credentials))
 
+  def test_unicode_header_checks(self):
+    access_token = u'foo'
+    client_id = u'some_client_id'
+    client_secret = u'cOuDdkfjxxnv+'
+    refresh_token = u'1/0/a.df219fjls0'
+    token_expiry = str(datetime.datetime.utcnow())
+    token_uri = str(GOOGLE_TOKEN_URI)
+    revoke_uri = str(GOOGLE_REVOKE_URI)
+    user_agent = u'refresh_checker/1.0'
+    credentials = OAuth2Credentials(access_token, client_id, client_secret,
+                                    refresh_token, token_expiry, token_uri,
+                                    user_agent, revoke_uri=revoke_uri)
+
+    # First, test that we correctly encode basic objects, making sure
+    # to include a bytes object. Note that oauth2client will normalize
+    # everything to bytes, no matter what python version we're in.
+    http = credentials.authorize(HttpMock(headers={'status': '200'}))
+    headers = {u'foo': 3, b'bar': True, 'baz': b'abc'}
+    cleaned_headers = {b'foo': b'3', b'bar': b'True', b'baz': b'abc'}
+    http.request(u'http://example.com', method=u'GET', headers=headers)
+    for k, v in cleaned_headers.items():
+      self.assertTrue(k in http.headers)
+      self.assertEqual(v, http.headers[k])
+
+    # Next, test that we do fail on unicode.
+    unicode_str = six.unichr(40960) + 'abcd'
+    self.assertRaises(
+        NonAsciiHeaderError,
+        http.request,
+        u'http://example.com', method=u'GET', headers={u'foo': unicode_str})
+
   def test_no_unicode_in_request_params(self):
     access_token = u'foo'
     client_id = u'some_client_id'