Can not use string offset as an array

The following code works well in PHP but in PHP 7 it gives an error:

  

Can not use string offset as an array

How can I solve the line with error in the following code?

if (empty($node['courses'])) {
    return '';
}

$result = '<ul>';
$coursecount = 0;
foreach ($node['courses'] as $course) {
    if (empty($course['certs'])) {
        continue;
        if (!isset($result['courses'][$coursename])) {
            $result['courses'][$coursename] = array('name' => $coursename, 'certs' => array());
        }

        if (!isset($result['course'][$coursename]['certs'][$issuecert->code])) {
            $result['courses'][$coursename]['certs'][$issuecert->code] = $issuecert; 
        }
    }
}