diff --git a/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerDeprecatedTest.php b/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerDeprecatedTest.php
index 1aa381fd09..8bdaf51c31 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerDeprecatedTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ModuleHandlerDeprecatedTest.php
@@ -12,6 +12,7 @@
  * so that we can mark it as belonging to the legacy group.
  *
  * @coversDefaultClass \Drupal\Core\Extension\ModuleHandler
+ * @runTestsInSeparateProcesses
  *
  * @group Extension
  * @group legacy