Invoking async.series inside async.series produces unpredictable output

Using caolan's async library for node.js, I've been trying to call a function that uses async.series inside another function that uses async.series, but I still can't get the functions to run in the correct order, as detailed below:

The terminal output shows the second function being called before the first, for no apparent reason:

The "sys" module is now called "util". It should have a similar interface.
Starting the second step in the series
Value of b: undefined
Invoking the function firstStep
the value of toObtain is: [object Object]

And here's the corresponding source code: 

var im = require('imagemagick');
var async = require('async');

var toObtain;


var b;
async.series([

function (callback) {
    //It appears that this function is being invoked after the second function.
    //Why is this happening?
    firstStep();
    callback();
},

function (callback) {
    //Why is the output of this function being displayed BEFORE the output of the function above? It's the opposite of the order in which I'm calling the functions.
    console.log("Starting the second step in the series");
    console.log("Value of b: " + b);
}]);


function firstStep(){
    async.series([

    function (next) { // step one - call the function that sets toObtain
        im.identify('kittens.png', function (err, features) {
            if (err) throw err;
            console.log("Invoking the function firstStep");
            toObtain = features;
            //console.log(toObtain);
            b = toObtain.height;
            next(); // invoke the callback provided by async
        });
    },

    function (next) { // step two - display it
        console.log('the value of toObtain is: %s',toObtain.toString());
    }]);
}

After about an hour of experimentation, I got it to work properly. I simply modified the firstStep function so that it takes a callback function as a parameter, and calls the callback at the end of the firstStep function.

var im = require('imagemagick');
var async = require('async');

var toObtain = false;


var b;
async.series([

function (callback) {
    firstStep(callback); //the firstStep function takes a callback parameter and calls the callback when it finishes running. Now everything seems to be working as intended.
},

function (callback) {
    console.log("Starting the second step in the series");
    console.log("Value of b: " + b);
}]);


function firstStep(theCallback){
    async.series([

    function (next) { // step one - call the function that sets toObtain
        im.identify('kittens.png', function (err, features) {
            if (err) throw err;
            console.log("Invoking the function firstStep");
            toObtain = features;
            //console.log(toObtain);
            b = toObtain.height;
            next(); // invoke the callback provided by async
        });
    },

    function (next) { // step two - display it
        console.log('the value of toObtain is: %s',toObtain.toString());
        theCallback();
    }]);
}