Illegal string offset

I have an error with this plugin

Warning: Illegal string offset ‘formId’ in C:\xampp\htdocs\wp-content\plugins\repeater-add-on-for-gravity-forms\class-gf-field-repeater.php on line 287

Warning: Illegal string offset ‘children’ in C:\xampp\htdocs\wp-content\plugins\repeater-add-on-for-gravity-forms\class-gf-field-repeater.php on line 291

if (empty($value)) {
$value[‘formId’] = $form_id;
if (!empty($repeater_start)) { $value[‘start’] = $repeater_start; }
if (!empty($repeater_min)) { $value[‘min’] = $repeater_min; }
if (!empty($repeater_max)) { $value[‘max’] = $repeater_max; }
if (!empty($repeater_children)) { $value[‘children’] = $repeater_children; }

$value = json_encode($value);
}