TypeError: Object #<Object> has no method 'apply'

[mishaaq]

Minimal example:

var AModel = Backbone.Model.extend({
    mutators: {
        foo: {
            set: function(key, value, options, set) {
                set(key, 'bar', options);
            }
        }
    },
    defaults: {
        foo: 'baz'
    }
 });

Calling (new AModel()).toJSON() results with error as above.
In the function Mutator.prototype.toJSON there is an assumption the mutator has always get method defined (under 'get' key of the mutator object or the mutator is a 'getter' itself). This is obviously wrong and such a check should be added:

if (_.isObject(mutator) === true && _.isFunction(mutator.get)) {
    attr[name] = _.bind(mutator.get, this)();
} else if (_.isFunction(mutator)) {
    attr[name] = _.bind(mutator, this)();
}

[asciidisco]

Yep. I hit that problem recently.
That was one of the reasons I avoided to reference 'this' within a mutator function to the model itself.
Would you like to put together a pull request?