code: function handleErrors(group, val) {
    if (utils.isObject(val)) {
      var List = group.List;
      var keys = Object.keys(List.prototype);
  
      keys.forEach(function(key) {
        if (typeof val[key] !== 'undefined') return;
  
        utils.define(val, key, function() {
          throw new Error(key + ' can only be used with an array of `List` items.');
        });
      });
    }
  }
docs: When `get` returns a non-Array object, we decorate
  noop `List` methods onto the object to throw errors when list methods
  are used, since list array methods do not work on groups.
  
  @param {Object} `group`
  @param {Object} `val` Value returned from `group.get()`