diff --git a/flask_cors/core.py b/flask_cors/core.py
index b37c9b3..c46f72d 100644
--- a/flask_cors/core.py
+++ b/flask_cors/core.py
@@ -13,7 +13,7 @@
 from datetime import timedelta
 from six import string_types
 from flask import request, current_app
-from werkzeug.datastructures import MultiDict
+from werkzeug.datastructures import Headers, MultiDict
 
 LOG = logging.getLogger(__name__)
 
@@ -226,6 +226,13 @@ def set_cors_headers(resp, options):
         LOG.debug('CORS have been already evaluated, skipping')
         return resp
 
+    # Some libraries, like OAuthlib, set resp.headers to non Multidict
+    # objects (Werkzeug Headers work as well). This is a problem because
+    # headers allow repeated values.
+    if (not isinstance(resp.headers, Headers)
+           and not isinstance(resp.headers, MultiDict)):
+        resp.headers = MultiDict(resp.headers)
+
     headers_to_set = get_cors_headers(options, request.headers, request.method)
 
     LOG.debug('Settings CORS headers: %s', str(headers_to_set))
diff --git a/tests/core/test_override_headers.py b/tests/core/test_override_headers.py
new file mode 100644
index 0000000..f76a1cd
--- /dev/null
+++ b/tests/core/test_override_headers.py
@@ -0,0 +1,33 @@
+# -*- coding: utf-8 -*-
+"""
+    test
+    ~~~~
+
+    Flask-Cors tests module
+"""
+
+from ..base_test import FlaskCorsTestCase
+from flask import Flask, Response
+
+from flask_cors import *
+from flask_cors.core import *
+
+class ResponseHeadersOverrideTestCaseIntegration(FlaskCorsTestCase):
+    def setUp(self):
+        self.app = Flask(__name__)
+        CORS(self.app)
+
+        @self.app.route('/')
+        def index():
+            response = Response(headers={"custom": "dictionary"})
+            return 'Welcome'
+
+    def test_override_headers(self):
+        '''
+            Ensure we work even if response.headers is set to something other than a MultiDict.
+        '''
+        for resp in self.iter_responses('/'):
+            self.assertTrue(ACL_ORIGIN in resp.headers)
+
+if __name__ == "__main__":
+    unittest.main()