Recently after updating Ruby on Rails, after runnig Webrick in a development mode, it started to crush with such an error:

 ERROR NoMethodError: undefined method `split' for nil:NilClass
    /home/path/gems/rack-1.4.3/lib/rack/handler/webrick.rb:68:in `block in service'
    /home/path/gems/rack-1.4.3/lib/rack/utils.rb:387:in `block in each'
    /home/path/gems/rack-1.4.3/lib/rack/utils.rb:386:in `each'
    /home/path/gems/rack-1.4.3/lib/rack/utils.rb:386:in `each'
    /home/path/gems/rack-1.4.3/lib/rack/handler/webrick.rb:62:in `service'
    /home/path/ruby-1.9.3-p194/lib/ruby/1.9.1/webrick/httpserver.rb:138:in `service'
    /home/path/ruby-1.9.3-p194/lib/ruby/1.9.1/webrick/httpserver.rb:94:in `run'
    /home/path/ruby-1.9.3-p194/lib/ruby/1.9.1/webrick/server.rb:191:in `block in start_thread'

I don’t know why it happens but I know how to fix it ;)
To do so, you need to find 68 line in a Webric server:

# Ofc change the path to your Webrick path
vim /home/path/gems/rack-1.4.3/lib/rack/handler/webrick.rb

Localize line 68 and change:

res[k] = vs.split("\n").join(", ")

to:

res[k] = vs.to_s.split("\n").join(", ")

It seams that the header value is somehow nil, that’s why we cast it to empty string.