diff --git a/tests/Material/Queue.php b/tests/Material/Queue.php
index 1f1e033bdd3b97e211cef619cafddd24451bff1c..5adc60ef683b33a261b66bcac1cb8658ebf01e96 100644
--- a/tests/Material/Queue.php
+++ b/tests/Material/Queue.php
@@ -18,7 +18,7 @@ class Queue
 
     public static function push(string $name, $value): int
     {
-        if (!is_array(self::$data[$name])) {
+        if (!is_array(self::$data[$name] ?? null)) {
             self::$data[$name] = [];
         }
         return array_push(self::$data[$name], $value);
@@ -26,7 +26,7 @@ class Queue
 
     public static function shift(string $name)
     {
-        if (!is_array(self::$data[$name])) {
+        if (!is_array(self::$data[$name] ?? null)) {
             return null;
         }
         return array_shift(self::$data[$name]);