diff --git a/assets/controllers/login_controller.js b/assets/controllers/login_controller.js
index b73a1cf..a7f4568 100644
--- a/assets/controllers/login_controller.js
+++ b/assets/controllers/login_controller.js
@@ -2,10 +2,25 @@ import { Controller } from '@hotwired/stimulus';
 import { getComponent } from '@symfony/ux-live-component';
 
 export default class extends Controller {
+  static targets = ['nostrError'];
+
   async initialize() {
     this.component = await getComponent(this.element);
   }
-  async loginAct() {
+  async loginAct(event) {
+    if (!window.nostr) {
+      if (this.hasNostrErrorTarget) {
+        this.nostrErrorTarget.textContent = 'Extension is not available.';
+        this.nostrErrorTarget.style.display = 'block';
+      }
+      event?.preventDefault();
+      return;
+    }
+    if (this.hasNostrErrorTarget) {
+      this.nostrErrorTarget.textContent = '';
+      this.nostrErrorTarget.style.display = 'none';
+    }
+
     const tags = [
       ['u', window.location.origin + '/login'],
       ['method', 'POST'],
@@ -35,4 +50,3 @@ export default class extends Controller {
     }
   }
 }
-
diff --git a/templates/components/UserMenu.html.twig b/templates/components/UserMenu.html.twig
index 7131f91..25ac226 100644
--- a/templates/components/UserMenu.html.twig
+++ b/templates/components/UserMenu.html.twig
@@ -31,6 +31,7 @@
                 </span>
                 <ul>
                     <li>
+                        <div data-login-target="nostrError" class="nostr-error-message" style="display:none;color:#b00;margin-bottom:0.5em;"></div>
                         <twig:Atoms:Button {{ ...stimulus_action('login', 'loginAct') }} tag="a" variant="accent">Extension</twig:Atoms:Button>
                     </li>
                     <li>