Sometimes, I have complex paths with maps and lists when using updateIn, and noticed something weird if passing a string instead of a number for a List index:

Immutable.List([]).setIn(['a'], 2).toJS()

returns []

Immutable.List([]).setIn(['123'], 2).toJS()

returns Array of size [1231]

Not sure best solution, buy perhaps way to go would be to try coerce string to number or only accept number for the indices and throw an exception.