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
  get '/' do
    "Hello World!\n"
  end
end

run A

Gemfile

source 'https://rubygems.org'

gem 'rack'
gem 'grape'