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.