diff --git a/system/Router/RouteCollection.php b/system/Router/RouteCollection.php
index 82c6a6dd5bbe..36b8e4d52340 100644
--- a/system/Router/RouteCollection.php
+++ b/system/Router/RouteCollection.php
@@ -28,7 +28,9 @@ class RouteCollection implements RouteCollectionInterface
 {
     /**
      * The namespace to be added to any Controllers.
-     * Defaults to the global namespaces (\)
+     * Defaults to the global namespaces (\).
+     *
+     * This must have a trailing backslash (\).
      *
      * @var string
      */
@@ -288,7 +290,7 @@ public function __construct(FileLocator $locator, Modules $moduleConfig, Routing
         $this->httpHost = Services::request()->getServer('HTTP_HOST');
 
         // Setup based on config file. Let routes file override.
-        $this->defaultNamespace   = $routing->defaultNamespace;
+        $this->defaultNamespace   = rtrim($routing->defaultNamespace, '\\') . '\\';
         $this->defaultController  = $routing->defaultController;
         $this->defaultMethod      = $routing->defaultMethod;
         $this->translateURIDashes = $routing->translateURIDashes;
diff --git a/tests/system/Test/FeatureTestTraitTest.php b/tests/system/Test/FeatureTestTraitTest.php
index 0f5bb7ce33d5..aee912281c76 100644
--- a/tests/system/Test/FeatureTestTraitTest.php
+++ b/tests/system/Test/FeatureTestTraitTest.php
@@ -11,9 +11,11 @@
 
 namespace CodeIgniter\Test;
 
+use CodeIgniter\Config\Factories;
 use CodeIgniter\Events\Events;
 use CodeIgniter\Exceptions\PageNotFoundException;
 use CodeIgniter\HTTP\Response;
+use Config\Routing;
 use Config\Services;
 
 /**
@@ -616,4 +618,15 @@ public function testSetupRequestBodyWithBody(): void
 
         $this->assertSame('test', $request->getBody());
     }
+
+    public function testAutoRoutingLegacy()
+    {
+        $config            = config(Routing::class);
+        $config->autoRoute = true;
+        Factories::injectMock('config', Routing::class, $config);
+
+        $response = $this->get('home/index');
+
+        $response->assertOK();
+    }
 }