uninitialized constant RubyCleanCSS::Exports::Http::OpenSSL (V8::Error)

[konsolebox]

I'm getting this error, probably with RubyCleanCSS::Compressor#compress:

/home/user/.gem/ruby/2.2.0/gems/ruby-clean-css-1.0.0/lib/ruby-clean-css/exports.rb:202:in `block in initialize': uninitialized constant RubyCleanCSS::Exports::Http::OpenSSL (V8::Error)
        from at Inliner.inlineRemoteResource (/home/user/.gem/ruby/2.2.0/gems/ruby-clean-css-1.0.0/lib/javascript/clean-css/lib/imports/inliner.js:224:5)
        from at Inliner.inline (/home/user/.gem/ruby/2.2.0/gems/ruby-clean-css-1.0.0/lib/javascript/clean-css/lib/imports/inliner.js:191:12)
        from at Inliner.process (/home/user/.gem/ruby/2.2.0/gems/ruby-clean-css-1.0.0/lib/javascript/clean-css/lib/imports/inliner.js:71:9)
        from at /home/user/.gem/ruby/2.2.0/gems/ruby-clean-css-1.0.0/lib/javascript/clean-css/lib/clean.js:65:63
        from /home/user/.gem/ruby/2.2.0/gems/ruby-clean-css-1.0.0/lib/ruby-clean-css/exports.rb:22:in `block in nextTick'
        from at CleanCSS.minify (/home/user/.gem/ruby/2.2.0/gems/ruby-clean-css-1.0.0/lib/javascript/clean-css/lib/clean.js:64:12)
        from /home/user/.gem/ruby/2.2.0/gems/ruby-clean-css-1.0.0/lib/ruby-clean-css/compressor.rb:16:in `compress'
        from ../_tools/compress-css.rb:28:in `block in main'
        from ../_tools/compress-css.rb:26:in `each'
        from ../_tools/compress-css.rb:26:in `main'
        from ../_tools/compress-css.rb:33:in `<main>'

But adding require 'openssl' to ruby-clean-css/exports.rb fixes it. I'm using ruby 2.0, 2.1 and 2.2.