Invalid argument supplied for foreach () ... Error

I have the following problem on my website, when I check the file "error_log" this em shows the following error:

"Invalid argument supplied for foreach () in / home / ..."

This is in the error location:

<?php                                       

        foreach ($on['result'] as $val) { ?>

            <a href="#" onClick="goToProfile(<?= $val['id'];?>)"><li class="chat__human" style="margin-bottom: 5px;">

                <img class="chat__avatar" style="padding: 1px;margin:3px" src="<?= $val['photo'];?>" alt="" />

                <span class="chat__name comforta" style="position: absolute;margin-left:5px;margin-top: 5px;font-weight: bold"><?= $val['firstName'];?></span>

                <span class="chat__last_m" style="position: absolute;margin-left:5px;margin-top: 23px;color:#999"> <?= $val['last_m'];?></span>

            </li></a>             

        <?php } ?>

I hope you can help me solve this error, thank you.