syck bug

These days, I am working on symbol parsing.
I found a bug of syck.
In the document of ruby,

  YAML.parse( "--- :locked" )
    #=> #          @type_id="tag:ruby.yaml.org,2002:sym",
          @value=":locked", @kind=:scalar>

but in fact,

irb(main):002:0> YAML.parse("--- :symbol").type_id
=> "tag:yaml.org,2002:str"