diff --git a/lib/rack/accept/response.rb b/lib/rack/accept/response.rb
index 88eddf0..c4cc006 100644
--- a/lib/rack/accept/response.rb
+++ b/lib/rack/accept/response.rb
@@ -12,7 +12,7 @@ class Response < Rack::Response
     def not_acceptable!
       self.status = 406
       self.body = []
-      header['Content-Length'] = '0'
+      headers['Content-Length'] = '0'
     end
   end
 end