How do I resolve the "undefined array key" error in PHP?

Question

I have an admin email check system as part of a wordpress plugin (wordpress database) and I am trying to let it work Warning: Undefined array key 1 on line (number)

Here is the code -

// Check for admin mail
    $admin_mail = $link->query("SELECT * FROM vragen");
    while ($rowadm = $admin_mail->fetch_array()) {
        if ($rowadm[1] == $email){
            $admin = '1';
        } 
    }

I tried a lot of things especially with the isset part of the php code

a lot of things - especially with the isset part of the php code