Error trying to convert from a subclassed dimension unit to a parent unit or viceversa

[waiting-for-dev]

class Screen < Quantity::Dimension::Length; end;
Screen.add_dimension :screen

Quantity::Unit.add_unit :px, :screen, 233333, :pixel, :pixels

puts 3.cm.to_px

Returns:

/var/lib/gems/1.9.1/gems/quantity-0.1.2/lib/quantity/unit.rb:236:in `convert': Cannot convert #<Quantity::Unit:0x000000015f68c8> to cm (ArgumentError)
        from /var/lib/gems/1.9.1/gems/quantity-0.1.2/lib/quantity.rb:331:in `convert'
        from /var/lib/gems/1.9.1/gems/quantity-0.1.2/lib/quantity.rb:354:in `method_missing'