Cannot read property push of undefined when combining arrays

Question

When pushing an array's contents to another array I get

var order = new Object(), stack = [];
for(var i=0;i<a.length;i++){
    if(parseInt(a[i].daysleft) == 0){ order[0].push(a[i]); }
    if(parseInt(a[i].daysleft) > 0){ order[1].push(a[i]); }
    if(parseInt(a[i].daysleft) < 0){ order[2].push(a[i]); }
}

Why do I get this error in the second if statement?