diff --git a/requests/models.py b/requests/models.py
index 4cf69b57..49c6364c 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -99,11 +99,13 @@ class RequestEncodingMixin(object):
         files = to_key_val_list(files or {})
 
         for field, val in fields:
-            if isinstance(val, list):
-                for v in val:
-                    new_fields.append((field, builtin_str(v)))
-            else:
-                new_fields.append((field, builtin_str(val)))
+            if isinstance(val, basestring) or not hasattr(val, '__iter__'):
+                val = [val]
+            for v in val:
+                if v is not None:
+                    new_fields.append(
+                        (field.encode('utf-8') if isinstance(field, str) else field,
+                         v.encode('utf-8') if isinstance(v, str) else v))
 
         for (k, v) in files:
             # support for explicit filename
diff --git a/test_requests.py b/test_requests.py
index 5440347a..e4c38c45 100644
--- a/test_requests.py
+++ b/test_requests.py
@@ -299,6 +299,27 @@ class RequestsTestCase(unittest.TestCase):
                           files={'file': ('test_requests.py', open(__file__, 'rb'))})
         self.assertEqual(r.status_code, 200)
 
+    def test_unicode_multipart_post(self):
+        r = requests.post(httpbin('post'),
+                          data={'stuff': u'ëlïxr'},
+                          files={'file': ('test_requests.py', open(__file__, 'rb'))})
+        self.assertEqual(r.status_code, 200)
+
+        r = requests.post(httpbin('post'),
+                          data={'stuff': u'ëlïxr'.encode('utf-8')},
+                          files={'file': ('test_requests.py', open(__file__, 'rb'))})
+        self.assertEqual(r.status_code, 200)
+
+        r = requests.post(httpbin('post'),
+                          data={'stuff': 'elixr'},
+                          files={'file': ('test_requests.py', open(__file__, 'rb'))})
+        self.assertEqual(r.status_code, 200)
+
+        r = requests.post(httpbin('post'),
+                          data={'stuff': 'elixr'.encode('utf-8')},
+                          files={'file': ('test_requests.py', open(__file__, 'rb'))})
+        self.assertEqual(r.status_code, 200)
+
     def test_custom_content_type(self):
         r = requests.post(httpbin('post'),
                           data={'stuff': json.dumps({'a': 123})},