diff --git a/oauth2/__init__.py b/oauth2/__init__.py
index 38bf520b..159d0b82 100644
--- a/oauth2/__init__.py
+++ b/oauth2/__init__.py
@@ -355,6 +355,8 @@ def __init__(self, method=HTTP_METHOD, url=None, parameters=None,
                 v = to_unicode_optional_iterator(v)
 
                 self[k] = v
+        if not isinstance(body, bytes):
+            body = body.encode('utf-8')
         self.body = body
         self.is_form_encoded = is_form_encoded