setAlpha() error

[dkrym]

Hi guys,
does anybody have similar problem? This exception happens time to time randomly. At first, I thought the problem is because of too many pages but it isn't. I generate file with 25-50 pages, all same layout with 1 small picture on every page (logo). I tried both jpg and png image. I guess it is image related but don't understand why sometimes I have this problem but sometime no problem at all.

edit: I found out that whenever I got this error, I can just delete my app cache folder and suddenly it works. Once. And then this error again.

I would appreciate any help or hint.
Thanks!

[2016-08-18 18:38:14] request.CRITICAL: Uncaught PHP Exception Symfony\Component\Debug\Exception\FatalErrorException: "Error: Call to a member function setAlpha() on null" at /home/brick/public_html/orders/vendor/psliwa/php-pdf/lib/PHPPdf/Core/ComplexAttribute/ComplexAttribute.php line 92 {"exception":"[object] (Symfony\\Component\\Debug\\Exception\\FatalErrorException(code: 0): Error: Call to a member function setAlpha() on null at /home/brick/public_html/orders/vendor/psliwa/php-pdf/lib/PHPPdf/Core/ComplexAttribute/ComplexAttribute.php:92)"} []