PHP Warning : A non-numeric value encountered

I get this Warning: A non-numeric value encountered in /home/customer/www/example.com/public_html/wp-content/themes/boombox/includes/functions.php on line 2466

I try to change it from => $manual_amount + $user_votes; to $manual_amount += $user_votes; (it throws same warning) and $manual_amount . $user_votes; (it concatenates the numbers, does not add them)

 /**
 * Return post point count
 *
 * @param $post_id
 *
 * @return int
 */
function boombox_get_post_point_count($post_id)
{
    $points_count = 0;
    /*if ( boombox_module_management_service()->is_module_active( 'prs' ) ) {
        $points_count = Boombox_Point_Count_Helper::get_post_points( $post_id );
    }*/
    $manual_amount = get_post_meta($post_id, "manual_vote_amount", true);
    $user_votes = get_post_meta($post_id, "add_vote_amount", true);
    $points_count = $manual_amount + $user_votes;
    return $points_count;
}

I'd appreciate your help