diff --git a/hack2-contrib.cabal b/hack2-contrib.cabal
--- a/hack2-contrib.cabal
+++ b/hack2-contrib.cabal
@@ -1,5 +1,5 @@
 Name:                 hack2-contrib
-Version:              2012.1.19.1
+Version:              2012.5.15
 Build-type:           Simple
 Synopsis:             Hack2 contrib
 Description:          Common middlewares and utilities that helps working with Hack2
diff --git a/src/Hack2/Contrib/Response.hs b/src/Hack2/Contrib/Response.hs
--- a/src/Hack2/Contrib/Response.hs
+++ b/src/Hack2/Contrib/Response.hs
@@ -44,7 +44,7 @@
 set_content_type :: ByteString -> Response -> Response
 set_content_type s r = r.set_header _ContentType s
 
-set_content_length :: (Integral a) => a -> Response -> Response
+set_content_length :: (Integral a, Show a) => a -> Response -> Response
 set_content_length i r = r.set_header _ContentLength (i.show_bytestring)
 
 set_body :: ByteString -> Response -> Response