Rightjs Core

This is part of my code. I store string value but get it as number

value = "item4";
id = value.substring(4);
alert(typeof(id)); // string
Cookie.set('item',id,{'path':'/'});
.
.
.
id = Cookie.get('item');
alert(typeof(id)); // number

a part from documentation:

Cookie.get(String name) -> String value or null

Is it bug or documentation error?