PdfResponse Only variables should be passed by reference

Zkouším tento doplněk na Nette 2.2.3 a bohužel se mi to nedaří. Vyskakuje na mě chyba:

Only variables should be passed by reference

Konkrétně mě to ukazuje chybu v /vendor/mpdf/mpdf/mpdf.php na řádku 18589.

 // mPDF 5.7.3 Inline tags
18581:        if ($tag=='PROGRESS'  || $tag=='METER') {
18582:            if (isset($this->InlineProperties[$tag]) && $this->InlineProperties[$tag]) { $this->restoreInlineProperties($this->InlineProperties[$tag]); }
18583:            unset($this->InlineProperties[$tag]);
18584:            if (isset($this->InlineAnnots[$tag]) && $this->InlineAnnots[$tag]) { $annot = $this->InlineAnnots[$tag]; }    // *ANNOTATIONS*
18585:            unset($this->InlineAnnots[$tag]);    // *ANNOTATIONS*
18586:        }
18587:        else {
18588:            if (isset($this->InlineProperties[$tag]) && count($this->InlineProperties[$tag])) {
18589:                $this->restoreInlineProperties(array_pop($this->InlineProperties[$tag]));

Kód vypadá následovně:

...
$pdf = new \PdfResponse\PdfResponse($template);
		$pdf->pageFormat = "A4";
        $pdf->documentAuthor = "Author";
		$pdf->documentTitle = "Name";
		$pdf->outputName = \Nette\Environment::expand("%wwwDir%") . "/invoices/" . $number . ".pdf";
		$pdf->outputDestination = "F";
		$this->presenter->sendResponse($pdf);