diff --git a/src/FileRouter.php b/src/FileRouter.php
index cdc498d..4fc90f0 100644
--- a/src/FileRouter.php
+++ b/src/FileRouter.php
@@ -129,16 +129,20 @@ public function process(ServerRequestInterface $request, RequestHandlerInterface
 
     private function parseRequestPath(ServerRequestInterface $request): iterable
     {
-        $possibleAction = null;
         $path = urldecode($request->getUri()->getPath());
 
-        if ($this->routePrefix !== '' && str_starts_with($path, $this->routePrefix)) {
-            $path = mb_substr($path, strlen($this->routePrefix));
+        if ($this->routePrefix !== '') {
+            if (!str_starts_with($path, $this->routePrefix)) {
+                return;
+            }
+            $path = mb_substr($path, mb_strlen($this->routePrefix));
             if ($path === '') {
                 $path = '/';
             }
         }
 
+        $possibleAction = null;
+
         if ($path === '/') {
             yield [
                 $this->cleanClassname(
diff --git a/tests/FileRouterTest.php b/tests/FileRouterTest.php
index 88df990..28d6f74 100644
--- a/tests/FileRouterTest.php
+++ b/tests/FileRouterTest.php
@@ -377,6 +377,25 @@ public static function dataModularity(): iterable
         ];
     }
 
+    public function testModularityFastPath(): void
+    {
+        $router = $this->createRouter();
+        $router = $router
+            ->withNamespace('Yiisoft\\FileRouter\\Tests\\Support\\App5\\Module1')
+            ->withRoutePrefix('/module1');
+
+
+        $handler = $this->createExceptionHandler();
+        $request = new ServerRequest(
+            method: 'GET',
+            uri: '/module2',
+        );
+
+        $this->expectException(\Exception::class);
+        $this->expectExceptionMessage('Not implemented from tests.');
+        $router->process($request, $handler);
+    }
+
     private function createRouter(): FileRouter
     {
         $container = new SimpleContainer([