If an object already has an attribute that is not an array/object and is deep extended to make that attribute an array, it converts the array into an object. For example:

var x = {y: 1};

$.extend(true, x, {y: [2, 3]});

alert(x.y); // x.y = {0: 2, 1: 3}

To fix, check that the source is already an array/object before trying to use when recursively extending. See line 18 in the attached code extract.