diff --git a/ceilometer/api/middleware.py b/ceilometer/api/middleware.py
index 3e9bfc676..2e04c5160 100644
--- a/ceilometer/api/middleware.py
+++ b/ceilometer/api/middleware.py
@@ -124,7 +124,7 @@ class ParsableErrorMiddleware(object):
                 except ValueError as err:
                     body = [json.dumps({'error_message': '\n'.join(app_iter)})]
                 state['headers'].append(('Content-Type', 'application/json'))
-            state['headers'].append(('Content-Length', len(body[0])))
+            state['headers'].append(('Content-Length', str(len(body[0]))))
         else:
             body = app_iter
         return body