diff --git a/requests/models.py b/requests/models.py
index 2727bee9..639565cf 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -81,7 +81,7 @@ class RequestEncodingMixin(object):
         """
 
         if isinstance(data, (str, bytes)):
-            return data
+            return to_native_string(data)
         elif hasattr(data, 'read'):
             return data
         elif hasattr(data, '__iter__'):
diff --git a/test_requests.py b/test_requests.py
index c5ac4f8d..7b23e594 100755
--- a/test_requests.py
+++ b/test_requests.py
@@ -139,6 +139,11 @@ class RequestsTestCase(unittest.TestCase):
         prep = session.prepare_request(request)
         assert prep.url == 'http://example.com/?z=1&a=1&k=1&d=1'
 
+    def test_params_bytes_are_encoded(self):
+        request = requests.Request('GET', 'http://example.com',
+                                   params=b'test=foo').prepare()
+        assert request.url == 'http://example.com/?test=foo'
+
     def test_mixed_case_scheme_acceptable(self):
         s = requests.Session()
         s.proxies = getproxies()