[SOLVED]Error: A non-numeric value encountered

I have this code:

PHP Code:

    public function pricesListByAjax()
    {
        if($this->request->isAJAX()){
            $html = ''; // Line 395
            $html += '<div>';
            $html += '<div class=""></div>';
            $html += '</div>';
            return $html;
        }
    } 


But in action I see this error:

PHP Code:

    "title": "ErrorException",
    "type": "ErrorException",
    "code": 500,
    "message": "A non-numeric value encountered",
    "file": "C:\\MAMP\\htdocs\\ci4\\app\\Controllers\\ProductController.php",
    "line": 395, 


how do can I fix this error?

See Line 395 in C:\\MAMP\\htdocs\\ci4\\app\\Controllers\\ProductController.php.

ci-phpunit-test | CodeIgniter Testing Guide | CodeIgniter 3 to 4 Upgrade Helper

See Line 395 in C:\\MAMP\\htdocs\\ci4\\app\\Controllers\\ProductController.php.

Line 395:

Code:

$html = ''; // Line 395

Not `$html += '<div>';`
But `$html .= '<div>';`

ci-phpunit-test | CodeIgniter Testing Guide | CodeIgniter 3 to 4 Upgrade Helper

Not `$html += '<div>';`
But `$html .= '<div>';`

Thanks Fix it.