diff --git a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
index b7848de..1314052 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
@@ -128,6 +128,12 @@ protected function setUp() {
     $modules = array_reverse($modules);
     $modules = call_user_func_array('array_merge_recursive', $modules);
     $this->enableModules($modules, FALSE);
+
+    // Re-initialize the theme to ensure that tests do not see an inconsistent
+    // behavior when calling functions that would initialize the theme if it has
+    // not been initialized yet.
+    drupal_theme_initialize();
+
     // In order to use theme functions default theme config needs to exist.
     \Drupal::config('system.theme')->set('default', 'stark');
   }
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index 265012e..2ee1d39 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -830,6 +830,11 @@ protected function setUp() {
       $this->rebuildContainer();
     }
 
+    // Re-initialize the theme to ensure that tests do not see an inconsistent
+    // behavior when calling functions that would initialize the theme if it has
+    // not been initialized yet.
+    drupal_theme_initialize();
+
     // Reset/rebuild all data structures after enabling the modules.
     $this->resetAll();