diff --git a/requests/models.py b/requests/models.py
index 730b9f5a..81ec2469 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -92,8 +92,10 @@ class RequestEncodingMixin(object):
         if parameters are supplied as a dict.
 
         """
-        if (not files) or isinstance(data, basestring):
-            return None
+        if (not files):
+            raise ValueError("Files must be provided.")
+        elif isinstance(data, basestring):
+            raise ValueError("Data must not be a string.")
 
         new_fields = []
         fields = to_key_val_list(data or {})
diff --git a/test_requests.py b/test_requests.py
index 27d5e676..01e17803 100755
--- a/test_requests.py
+++ b/test_requests.py
@@ -357,6 +357,12 @@ class RequestsTestCase(unittest.TestCase):
         except ValueError:
             pass
 
+        with open('requirements.txt') as f:
+            try:
+                requests.post(url, data='[{"some": "data"}]', files={'some': f})
+            except ValueError:
+                pass
+
     def test_request_ok_set(self):
         r = requests.get(httpbin('status', '404'))
         self.assertEqual(r.ok, False)