Laravel Undefined offset: 1451

I'm trying to sum some elements fetched from Database as Collection in a new created array and the code looks like

foreach ($demands as $demand) {

    $demandProject[$demand->projectid][$demand->week]['tickets'] += isset($demand->title) ? $demand->title : '';
    $demandProject[$demand->projectid][$demand->week]['total']   += (float) $demand->time;
    $demandProject[$demand->projectid][$demand->week]['l1']      += $demand->time_l1;
    $demandProject[$demand->projectid][$demand->week]['l2']      += $demand->time_l2;
    $demandProject[$demand->projectid][$demand->week]['l3']      += $demand->time_l3;
    $demandProject[$demand->projectid][$demand->week]['l4']      += $demand->time_l4;
    $demandProject[$demand->projectid][$demand->week]['l5']      += $demand->time_l5;
    $demandProject[$demand->projectid][$demand->week]['ns']      += $demand->time_ns;
}

but I'm getting

ErrorException
Undefined offset: 1451

what does that error mean what I'm doing wrong

via Chebli Mohamed