Why am I getting an infinite loop when I don't define the parameters?

I would like to know why I am getting an infinite loop here. I just don't want to pass this initial values, so if they are undefined they get automatically calculated. Its just to clean my function call to use only a single parameter. If I pass them everything runs ok and the process ends. Can anyone help? Thanks

function merge(array, lower, half, upper){
    //Suppressed for the sake of brevity
}

function mergeSort(array, lower, upper){
    if(!lower && !upper){ //take a look here
        lower = 0;
        upper = array.length - 1;
    }

    if(lower < upper){
        var half = Math.floor((lower + upper)/2);

        mergeSort(array, lower, half);
        mergeSort(array, half + 1, upper);
        merge(array, lower, half, upper);
    }
}

var array = [8, 3, 6, 4, 1, 0, 23, 12, 15, 63];
mergeSort(array); //infinite loop here
console.log(array);