diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php b/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
index 07bf39bf..0086f391 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php
@@ -1105,7 +1105,7 @@ class Worksheet extends WriterPart
                     break;
                 case 'f':            // Formula
                     $attributes = $pCell->getFormulaAttributes();
-                    if ($attributes['t'] === 'array') {
+                    if (($attributes['t'] ?? null) === 'array') {
                         $objWriter->startElement('f');
                         $objWriter->writeAttribute('t', 'array');
                         $objWriter->writeAttribute('ref', $pCellAddress);