Error Trying to access array offset on value of type bool

[ssgnoe]

Using on Lumen 6.0
´´´php
$pdfMerger = new \PDFMerger;
$pdfMerger->addPDF(storage_path('pdf.pdf'), 'all');
$pdfMerger->merge('browser');
´´´

Causes Error:
Trying to access array offset on value of type bool

PHP in Browser 7.4

What am I doing wrong? Any ideas?
Tested merge('file', 'test.pdf');
as well as string.