diff --git a/libraries/classes/Header.php b/libraries/classes/Header.php
index 42df457d79..59fe4bc2fa 100644
--- a/libraries/classes/Header.php
+++ b/libraries/classes/Header.php
@@ -709,7 +709,7 @@ class Header
         $maxInputVarsValue = $maxInputVars === false || $maxInputVars === '' ? 'false' : (int) $maxInputVars;
 
         return $this->template->render('javascript/variables', [
-            'first_day_of_calendar' => $cfg['FirstDayOfCalendar'],
+            'first_day_of_calendar' => $cfg['FirstDayOfCalendar'] ?? 0,
             'theme_image_path' => $PMA_Theme !== null ? $PMA_Theme->getImgPath() : '',
             'max_input_vars' => $maxInputVarsValue,
         ]);