diff --git a/prometheus_client/exposition.py b/prometheus_client/exposition.py
index 18233e24..2b437a69 100644
--- a/prometheus_client/exposition.py
+++ b/prometheus_client/exposition.py
@@ -122,7 +122,7 @@ def prometheus_app(environ, start_response):
             # Bake output
             status, header, output = _bake_output(registry, accept_header, params)
         # Return output
-        start_response(status, [header])
+        start_response(str(status), [header])
         return [output]
 
     return prometheus_app