TCPDI Uninitialized string offset: 35

[AlexRederstein]

Hello! I have a code with which I add a png image to a pdf document. But if you upload a document larger than 1MB, an error occurs: Uninitialized string offset: 35. Here is my code:
` $pdf = new TCPDI(PDF_PAGE_ORIENTATION, 'px', PDF_PAGE_FORMAT, true, 'UTF-8', false);
$lists = $pdf->setSourceFile($file_dir);
$pdf->SetAutoPageBreak(false);
for($i=1; $i<=$lists; $i++) {
$import = $pdf->importPage($i);
$size = $pdf->getTemplatesize($import);
$pdf->AddPage($size['w'] > $size['h'] ? 'L' : 'P');
$pdf->useTemplate($import, 0, 0);
if($i==$lists){
$res = getimagesize(stream_get_meta_data($this->sign)['uri']);
$pdf->SetY(-$res[1] / 2 - 10);
$pdf->SetX(-$res[0] / 2 - 10);
$pdf->Image(stream_get_meta_data($this->sign)['uri'], $pdf->getX(), $pdf->getY(), $res[0] / 2, $res[1] / 2);
}
}

	$tmp = tmpfile();
	$pdf->output('test.pdf','I');
	return base64_encode(file_get_contents(stream_get_meta_data($tmp)['uri']));

Php.ini config:max_execution_time = 60
max_input_vars = 5000
memory_limit = -1
post_max_size = 128M
upload_max_filesize = 50M
`