diff --git a/module-runtime.js b/module-runtime.js
index dabe0cd..0872172 100644
--- a/module-runtime.js
+++ b/module-runtime.js
@@ -28,7 +28,11 @@ const server = createModuleServer({
   logger,
   identityRequired: true,
   routes(request, response, context) {
-    const targetPath = `${basePath}${context.url.pathname}${context.url.search}`;
+    // Next.js treats the configured base path itself as the canonical root
+    // (without a trailing slash). Keep `/kalendartool` stable when MPM routes
+    // an incoming `/kalendartool` request as the module path `/`.
+    const modulePath = context.url.pathname === '/' ? '' : context.url.pathname;
+    const targetPath = `${basePath}${modulePath}${context.url.search}`;
     const headers = { ...request.headers, host: `127.0.0.1:${nextPort}` };
     for (const name of Object.keys(headers)) {
       if (name.startsWith('x-kalendartool-mpm-')) delete headers[name];