diff --git a/demos/_unit-test/callback-nested.php b/demos/_unit-test/callback-nested.php
index 5d4e9a569e..746313fe4c 100644
--- a/demos/_unit-test/callback-nested.php
+++ b/demos/_unit-test/callback-nested.php
@@ -10,6 +10,7 @@
 
 use Atk4\Ui\Button;
 use Atk4\Ui\Crud;
+use Atk4\Ui\Exception;
 use Atk4\Ui\Header;
 use Atk4\Ui\Loader;
 use Atk4\Ui\UserAction\ExecutorFactory;
@@ -25,26 +26,37 @@
 );
 
 $loader = Loader::addTo($app);
+$loader->cb->setUrlTrigger('trigger_main_loader');
 $loader->loadEvent = false;
 
-$loader->set(function ($p) use ($m) {
-    $loader_1 = Loader::addTo($p);
-    $loader_1->loadEvent = false;
-
+$loader->set(function (Loader $p) use ($m) {
     Header::addTo($p, ['Loader-1', 'size' => 4]);
 
-    $loader_1->set(function ($p) use ($m) {
+    if (isset($_GET['err_main_loader'])) {
+        throw new Exception('Exception from Main Loader');
+    }
+
+    $loaderSub = Loader::addTo($p);
+    $loaderSub->cb->setUrlTrigger('trigger_sub_loader');
+    $loaderSub->loadEvent = false;
+
+    $loaderSub->set(function (Loader $p) use ($m) {
         Header::addTo($p, ['Loader-2', 'size' => 4]);
-        $loader_3 = Loader::addTo($p);
 
-        $loader_3->set(function ($p) use ($m) {
+        if (isset($_GET['err_mid_loader'])) {
+            throw new Exception('Exception from Sub Loader');
+        }
+
+        $loaderSubSub = Loader::addTo($p);
+
+        $loaderSubSub->set(function (Loader $p) use ($m) {
             Header::addTo($p, ['Loader-3', 'size' => 4]);
 
             $c = Crud::addTo($p, ['ipp' => 4]);
             $c->setModel($m, [$m->fieldName()->name]);
         });
     });
-    \Atk4\Ui\Button::addTo($p, ['Load2'])->js('click', $loader_1->jsLoad());
+    \Atk4\Ui\Button::addTo($p, ['Load2'])->js('click', $loaderSub->jsLoad());
 });
 
 \Atk4\Ui\Button::addTo($app, ['Load1'])->js('click', $loader->jsLoad());
diff --git a/src/Loader.php b/src/Loader.php
index dbff967ea7..830fff5b7e 100644
--- a/src/Loader.php
+++ b/src/Loader.php
@@ -21,7 +21,7 @@ class Loader extends View
     public $shim;
 
     /**
-     * Specify which event will cause Loader to begen fetching it's actual data. In some cases
+     * Specify which event will cause Loader to begin fetching it's actual data. In some cases
      * you would want to wait. You can set a custom JavaScript event name then trigger() it.
      *
      * Default value is `true` which means loading will take place as soon as possible. Setting this
@@ -35,7 +35,7 @@ class Loader extends View
     public $ui = 'ui segment';
 
     /** @var Callback for triggering */
-    protected $cb;
+    public $cb;
 
     /** @var array Url arguments. */
     public $urlArgs = [];
diff --git a/tests/DemosTest.php b/tests/DemosTest.php
index 5c62bbd567..8179a5c92f 100644
--- a/tests/DemosTest.php
+++ b/tests/DemosTest.php
@@ -8,6 +8,7 @@
 use Atk4\Data\Persistence;
 use Atk4\Ui\App;
 use Atk4\Ui\Callback;
+use Atk4\Ui\Exception\UnhandledCallbackExceptionError;
 use GuzzleHttp\Client;
 use GuzzleHttp\Psr7\Request;
 use GuzzleHttp\Psr7\Response;
@@ -479,23 +480,44 @@ public function testDemoAssertJsonResponsePost(string $uri, array $postData): vo
         $this->assertMatchesRegularExpression($this->regexJson, $response->getBody()->getContents());
     }
 
-    public function testDemoCallbackError(): void
+    /**
+     * @dataProvider demoCallbackErrorProvider
+     */
+    public function testDemoCallbackError(string $uri, string $expectedExceptionMessage): void
     {
         if (static::class === self::class) {
             $this->expectException(\Atk4\Ui\Exception::class);
-            $this->expectExceptionMessage('Callback requested, but never reached. You may be missing some arguments in request URL.');
+            $this->expectExceptionMessage($expectedExceptionMessage);
         }
 
-        $uri = 'obsolete/notify2.php?' . Callback::URL_QUERY_TRIGGER_PREFIX . 'test_notify=ajax&' . Callback::URL_QUERY_TARGET . '=non_existing_target';
-
         try {
-            $response = $this->getResponseFromRequest($uri, ['form_params' => ['width' => '25%']]);
+            $response = $this->getResponseFromRequest($uri);
         } catch (\GuzzleHttp\Exception\ServerException $e) {
             $response = $e->getResponse();
+        } catch (UnhandledCallbackExceptionError $e) {
+            while ($e instanceof UnhandledCallbackExceptionError) {
+                $e = $e->getPrevious();
+            }
+
+            throw $e;
         }
 
         $this->assertSame(500, $response->getStatusCode());
-        $this->assertStringContainsString('Callback requested, but never reached. You may be missing some arguments in request URL.', $response->getBody()->getContents());
+        $this->assertStringContainsString($expectedExceptionMessage, $response->getBody()->getContents());
+    }
+
+    public function demoCallbackErrorProvider(): array
+    {
+        return [
+            [
+                '_unit-test/callback-nested.php?' . Callback::URL_QUERY_TRIGGER_PREFIX . 'trigger_main_loader=callback&' . Callback::URL_QUERY_TARGET . '=non_existing_target',
+                'Callback requested, but never reached. You may be missing some arguments in request URL.',
+            ],
+            [
+                '_unit-test/callback-nested.php?err_main_loader&' . Callback::URL_QUERY_TRIGGER_PREFIX . 'trigger_main_loader=callback&' . Callback::URL_QUERY_TARGET . '=trigger_main_loader',
+                'Exception from Main Loader',
+            ],
+        ];
     }
 }