diff --git a/oauth2/__init__.py b/oauth2/__init__.py
index a1776a75..64ba83c1 100644
--- a/oauth2/__init__.py
+++ b/oauth2/__init__.py
@@ -490,7 +490,7 @@ def sign_request(self, signature_method, consumer, token):
             # section 4.1.1 "OAuth Consumers MUST NOT include an
             # oauth_body_hash parameter on requests with form-encoded
             # request bodies."
-            self['oauth_body_hash'] = base64.b64encode(sha1(self.body).digest())
+            self['oauth_body_hash'] = base64.b64encode(sha1(str(self.body).encode('utf-8')).digest())
 
         if 'oauth_consumer_key' not in self:
             self['oauth_consumer_key'] = consumer.key