diff --git a/mwclient/http.py b/mwclient/http.py
index 8cc8507b6d5845525372d7fd99ee935182f92f8b..0a927a49fffb11518be7cff1f14200d83f02e40e 100644
--- a/mwclient/http.py
+++ b/mwclient/http.py
@@ -84,9 +84,9 @@ class HTTPPersistentConnection(object):
 			headers['Cookie'] = self.cookies[host].get_cookie_header()
 		if issubclass(data.__class__, upload.Upload):
 			headers['Content-Type'] = data.content_type
-			headers['Content-Length'] = data.length;
+			headers['Content-Length'] = str(data.length)
 		elif data:
-			headers['Content-Length'] = len(data)
+			headers['Content-Length'] = str(len(data))
 			
 		if _headers: headers.update(_headers)