This is the code that works just fine:

require 'sinatra'
post '/foo' do
  "Equals to #{params[:a]}!"
end

If I send POST request, it returns all right:

$ ruby foo.rb -p 8888
$ curl -X POST -H 'Content-Length: 5' --data 'a=444' http://localhost:8888/foo
Equals to 444!

Now, I modify the code:

require 'sinatra'
use Rack::RewindableInput::Middleware
post '/foo' do
  "Equals to #{params[:a]}!"
end

I’m getting this:

$ ruby foo.rb -p 8888
$ curl -X POST -H 'Content-Length: 5' --data 'a=444' http://localhost:8888/foo
Equals to !

What am I doing wrong?