gistfile1.rb

$ irb
>> class Foo; def to_yaml(options); 'abc'.to_yaml(options) end end
=> nil
>> { 'foo' => Foo.new }.to_yaml
=> "--- \nfoo: abc\n"

$ jirb
>> class Foo; def to_yaml(options); 'abc'.to_yaml(options) end end
=> nil
>> { 'foo' => Foo.new }.to_yaml                                   
TypeError: wrong argument type String (expected YAML::JvYAML::Node)
	from (irb):3