a_output.txt

Marie@cosmic-joystick MINGW64 ~/rubygems-issue-1392-testcase
$ rackup
C:/Ruby22/lib/ruby/gems/2.2.0/gems/rack-1.6.4/lib/rack/builder.rb:146:in `to_app': missing run or map statement (RuntimeError)
        from C:/Users/Marie/rubygems-issue-1392-testcase/config.ru:9:in `<main>'
        from C:/Ruby22/lib/ruby/gems/2.2.0/gems/rack-1.6.4/lib/rack/builder.rb:49:in `eval'
        from C:/Ruby22/lib/ruby/gems/2.2.0/gems/rack-1.6.4/lib/rack/builder.rb:49:in `new_from_string'
        from C:/Ruby22/lib/ruby/gems/2.2.0/gems/rack-1.6.4/lib/rack/builder.rb:40:in `parse_file'
        from C:/Ruby22/lib/ruby/gems/2.2.0/gems/rack-1.6.4/lib/rack/server.rb:299:in `build_app_and_options_from_config'
        from C:/Ruby22/lib/ruby/gems/2.2.0/gems/rack-1.6.4/lib/rack/server.rb:208:in `app'
        from C:/Ruby22/lib/ruby/gems/2.2.0/gems/rack-1.6.4/lib/rack/server.rb:336:in `wrapped_app'
        from C:/Ruby22/lib/ruby/gems/2.2.0/gems/rack-1.6.4/lib/rack/server.rb:272:in `start'
        from C:/Ruby22/lib/ruby/gems/2.2.0/gems/rack-1.6.4/lib/rack/server.rb:147:in `start'
        from C:/Ruby22/lib/ruby/gems/2.2.0/gems/rack-1.6.4/bin/rackup:4:in `<top (required)>'
        from C:/Ruby22/bin/rackup:23:in `load'
        from C:/Ruby22/bin/rackup:23:in `<main>'

Marie@cosmic-joystick MINGW64 ~/rubygems-issue-1392-testcase
$ 

config.ru

require 'grape'

class A < Grape::API
  format :json
  get '/' do
    { :message => "Hello World!\n" }
  end
end

run A

Gemfile

source 'https://rubygems.org'

gem 'rack'
gem 'grape'

j_output.txt

% rackup
[2016-03-08 10:51:39] INFO  WEBrick 1.3.1
[2016-03-08 10:51:39] INFO  ruby 2.3.0 (2015-12-25) [x86_64-darwin15]
[2016-03-08 10:51:39] INFO  WEBrick::HTTPServer#start: pid=39391 port=9292
::1 - - [08/Mar/2016:10:52:43 -0600] "GET / HTTP/1.1" 200 28 0.0908

% curl -v http://localhost:9292/
*   Trying ::1...
* Connected to localhost (::1) port 9292 (#0)
> GET / HTTP/1.1
> Host: localhost:9292
> User-Agent: curl/7.43.0
> Accept: */*
>
< HTTP/1.1 200 OK
< Content-Type: application/json
< Content-Length: 28
< Server: WEBrick/1.3.1 (Ruby/2.3.0/2015-12-25)
< Date: Tue, 08 Mar 2016 16:52:43 GMT
< Connection: Keep-Alive
<
* Connection #0 to host localhost left intact
{"message":"Hello World!\n"}