diff --git a/core/tests/Drupal/Tests/Core/Extension/ExtensionDiscoveryTest.php b/core/tests/Drupal/Tests/Core/Extension/ExtensionDiscoveryTest.php
index e70e05c0f51307f4b06982f5ed6c2d3ef7e84d01..d087b76b034b382b33f5d329f1cc1597a946cae3 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ExtensionDiscoveryTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ExtensionDiscoveryTest.php
@@ -127,6 +127,13 @@ protected function populateFilesystemStructure(array &$filesystem_structure) {
       'core/profiles/minimal/minimal.info.yml' => [
         'type' => 'profile',
       ],
+      'core/themes/test_theme/test_theme.info.yml' => [
+        'type' => 'theme',
+      ],
+      // Override the core instance of the 'test_theme' theme.
+      'sites/default/themes/test_theme/test_theme.info.yml' => [
+        'type' => 'theme',
+      ],
       // Override the core instance of the 'minimal' profile.
       'sites/default/profiles/minimal/minimal.info.yml' => [
         'type' => 'profile',