> a = i.Map().set(null, 1) > b = i.Map().set(null, 2) > id(a, b) param str (null) is not a string The error comes from https://github.com/intelie/immutable-js-diff/blob/master/src/path.js#L14 where it is trying to determine the path of the key (https://github.com/intelie/immutable-js-diff/blob/master/src/diff.js#L56).