diff --git a/src/PhpSpreadsheet/Reader/Xls.php b/src/PhpSpreadsheet/Reader/Xls.php
index aa8a5bc5..8a40f70b 100644
--- a/src/PhpSpreadsheet/Reader/Xls.php
+++ b/src/PhpSpreadsheet/Reader/Xls.php
@@ -7017,7 +7017,7 @@ class Xls extends BaseReader
                 // offset: 3; size: 2; one-based index to DEFINEDNAME or EXTERNNAME record
                 $index = self::getUInt2d($formulaData, 3);
                 // assume index is to EXTERNNAME record
-                $data = $this->externalNames[$index - 1]['name'];
+                $data = $this->externalNames[$index - 1]['name'] ?? '';
                 // offset: 5; size: 2; not used
                 break;
             case 0x3A:    //    3d reference to cell