TCPDF错误; 未定义的偏移量

I am new to php and html.I am currently developing a functioning bookstore website.In it the administrator needs to beable to download a pdf of the customer list. i have used the tcpdf library however i ran into an error while using it. The error indicates an undefined offset withing the library while using the writeHTML function. Please note this is the code for after you press the create pdf button. line67 writeHTML() gives an error of
Notice: Undefined offset: 0 in C:\wamp64\www\pdf2\tcpdf\tcpdf.php on line 17161 , 17528, 17981,17981, 18279, 17981, 18288, 18290, 17528,17528 –

if(isset($_POST["create_pdf"]))
{
    require_once("tcpdf/tcpdf.php");
$obj_pdf = new TCPDF('P',PDF_UNIT,PDF_PAGE_FORMAT,true,"UTF-8",false);
$obj_pdf->SetCreator(PDF_CREATOR);
$obj_pdf->SetTitle("Customer List");
$obj_pdf->SetHeaderData("","", PDF_HEADER_TITLE, PDF_HEADER_STRING);
$obj_pdf->SetHeaderFont(Array(PDF_FONT_NAME_MAIN,"",PDF_FONT_SIZE_MAIN));
$obj_pdf->SetFooterFont(Array(PDF_FONT_NAME_DATA,"",PDF_FONT_SIZE_DATA));
$obj_pdf->SetDefaultMonospacedFont('helvetica');
$obj_pdf->SetFooterMargin(PDF_MARGIN_FOOTER);
$obj_pdf->SetMargins(PDF_MARGIN_LEFT,'5',PDF_MARGIN_RIGHT);
$obj_pdf->SetPrintHeader(false);
$obj_pdf->SetPrintFooter(false);
$obj_pdf->SetAutoPageBreak(TRUE,10);
$obj_pdf->SetFont('helvetica',"",12);

$content="";
$content.='
    <h3 align="center"> Customer List </h3>
    <table border="1" cellspacing="0" cellpadding="5">
        <tr>
            <th width=5%>cus_id</th>
            <th width=10%>cus_f_name</th>
            <th width=10%>cus_surname</th>
            <th width=15%>cus_street_address</th>
            <th width=15%>cus_city</th>
            <th width=5%>cus_zip_code</th>
            <th width=10%>cus_country</th>
            <th width=15%>cus_phone_number</th>
            <th width=15%>cus_email</th>
        </tr>
';

$content .= fetch_data();

$content .= '</table>';

$obj_pdf->writeHTML($content);

$obj_pdf->Output("sample.pdf","I");