.map method on dom.List and dom.SExpression throws exception

[fnmdx111]

https://github.com/amzn/ion-js/blob/master/src/dom/Sequence.ts#L8 says Sequences extend Array, and since dom.List and dom.SExpression extends Sequence, I would expect them to support map for iterating the elements. However, this (https://runkit.com/embed/n9x2izs3st66) throws in 4.1.0.