Pass Array in MongoDB MapReduce scope

I'm working on a a MapReduce execution on my Node.js + Express.js server.

I'm using Mongoose as ODM, and I need to pass a key-value array named _affinities into the map function global scope. To do this, I'm currently passing the array inside the o.scope object:

var o = {}; // Object sent to Mongoose as options

// Scope to pass affinities
o.scope = {
    affs : _affinities,
    test : 'test'
}

However, while using those values in o.map, it says affs is an empty array. However, if I try to log test, it shows the correct value.

// Note: this is just a test, I don't need to pass affs & test as the key.
o.map = function() {
    emit({ 
       a : affs, 
       test : test,
       id : this._id 
    }, this.score);
}

I have only one finalize function (for testing), without a reduce one.

o.finalize = function (k, v) 
{
    return v;
}

MyModel.mapReduce(o, callback);

Am I doing something wrong? Is not supported by MongoDB passing arrays?