class-book.rb stack trace

gistfile1.txt

$ bin/shoes samples/class-book.rb
NoMethodError: undefined method `[]' for #<Shoes::Link:0x1304ff98>
         normalize_style at /home/philipp/code/shoes4/lib/shoes/dsl.rb:75
                    each at org/jruby/RubyArray.java:1613
         normalize_style at /home/philipp/code/shoes4/lib/shoes/dsl.rb:74
                    span at /home/philipp/code/shoes4/lib/shoes/dsl.rb:463
          method_missing at /home/philipp/code/shoes4/lib/shoes/url.rb:22
       table_of_contents at /home/philipp/code/shoes4/samples/class-book.rb:21
                incident at /home/philipp/code/shoes4/samples/class-book.rb:32
                    call at org/jruby/RubyProc.java:271
              eval_block at /home/philipp/code/shoes4/lib/shoes/slot.rb:46
              initialize at /home/philipp/code/shoes4/lib/shoes/slot.rb:18
                  create at /home/philipp/code/shoes4/lib/shoes/dsl.rb:86
                    flow at /home/philipp/code/shoes4/lib/shoes/dsl.rb:134
          method_missing at /home/philipp/code/shoes4/lib/shoes/url.rb:22
                incident at /home/philipp/code/shoes4/samples/class-book.rb:31
                   index at /home/philipp/code/shoes4/samples/class-book.rb:8
                     url at /home/philipp/code/shoes4/lib/shoes/url.rb:45
                    call at org/jruby/RubyProc.java:271
                   visit at /home/philipp/code/shoes4/lib/shoes/dsl.rb:504
                    call at org/jruby/RubyProc.java:271
              eval_block at /home/philipp/code/shoes4/lib/shoes/slot.rb:46
                   clear at /home/philipp/code/shoes4/lib/shoes/slot.rb:41
                   clear at /home/philipp/code/shoes4/lib/shoes/app.rb:114
             public_send at org/jruby/RubyKernel.java:1963
                   clear at /home/philipp/code/shoes4/lib/shoes/app.rb:66
                   visit at /home/philipp/code/shoes4/lib/shoes/dsl.rb:502
  create_execution_block at /home/philipp/code/shoes4/lib/shoes/app.rb:228
                    call at org/jruby/RubyProc.java:271
              eval_block at /home/philipp/code/shoes4/lib/shoes/slot.rb:46
                  append at /home/philipp/code/shoes4/lib/shoes/slot.rb:55
              eval_block at /home/philipp/code/shoes4/lib/shoes/app.rb:218
               setup_gui at /home/philipp/code/shoes4/lib/shoes/app.rb:102
              initialize at /home/philipp/code/shoes4/lib/shoes/app.rb:31
                     app at /home/philipp/code/shoes4/lib/shoes/app.rb:15
                  (root) at /home/philipp/code/shoes4/samples/class-book.rb:42
                    load at org/jruby/RubyKernel.java:1099
                  (root) at /home/philipp/code/shoes4/lib/shoes/cli.rb:1
             execute_app at /home/philipp/code/shoes4/lib/shoes/cli.rb:73
                  (root) at /home/philipp/code/shoes4/bin/ruby-shoes:6