diff --git a/core/modules/system/lib/Drupal/system/Tests/Module/ModuleTestBase.php b/core/modules/system/lib/Drupal/system/Tests/Module/ModuleTestBase.php
index f23204b..41d0d4c 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Module/ModuleTestBase.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Module/ModuleTestBase.php
@@ -105,9 +105,11 @@ function assertModuleConfig($module) {
     $this->assertTrue($names);
 
     // Look up each default configuration object name in the active
-    // configuration, and if it exists, remove it from the stack.
+    // configuration. If it exists or belongs to another module's namespace,
+    // remove it from the stack.
     foreach ($names as $key => $name) {
-      if (config($name)->get()) {
+      // @todo Replace the module check with a proper config method.
+      if ((strtok($name, '.') != $module) || config($name)->get()) {
         unset($names[$key]);
       }
     }