List.keySeq().last() always returns -1

With Map:

var map = Immutable.Map({one: 'one', two: 'two', three: 'three'});
map.keySeq().last();
// 'three'

With List though:

var list = Immutable.List(['one', 'two', 'three']);
list.keySeq().last();
// -1

Am I missing something or is this in fact a bug?

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