In file https://github.com/nesquena/rabl/blob/master/lib/rabl/template.rb there is this line of code in row 44 Rails.version =~ /^/
Rails.version =~ /^/
In Rails 4 the version parameter is no longer an string and need to be parsed with 'to_s'.
'to_s' should do no harm if it's already an string so this should also works in rails 3.
I may add an pull request later if i have time and no one is quicker than me.
[template] Convert rails version to string explicitly (@alex-ross) #433