What happened

List.insert makes the inserted object into an array in IE 11

IE version : 11.648.17134.0

I use IE 11 to open https://immutable-js.github.io/immutable-js/docs/#/List to test in the console.

How to reproduce

1. Defined an object list
2. Insert an object at the beginning of the list
3. The first object becomes an array

e.g.

var a= Immutable.fromJS({ a: 0 })
var b = Immutable.fromJS({ b: 0 })
var c = Immutable.fromJS({ c: 0 });
var list = Immutable.fromJS([b, c])

var newlist = list.insert(0, a); 
// expect [Object, Object, Object]
// actual:  [Array, Object, Object]

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