diff --git a/src/Bridge/Symfony/DependencyInjection/CompilerPass/MessageHandlersLocatorPass.php b/src/Bridge/Symfony/DependencyInjection/CompilerPass/MessageHandlersLocatorPass.php
index ba9bc78..9f069d3 100644
--- a/src/Bridge/Symfony/DependencyInjection/CompilerPass/MessageHandlersLocatorPass.php
+++ b/src/Bridge/Symfony/DependencyInjection/CompilerPass/MessageHandlersLocatorPass.php
@@ -47,7 +47,7 @@ public function process(ContainerBuilder $container): void
         if (!$this->allowMultiple) {
             foreach ($handlers as $class => $refs) {
                 if (count($refs) > 1) {
-                    throw new LogicException(sprintf('Only one handler is allowed for %s of type "%s".', $this->topic, $class));
+                    throw new LogicException(sprintf('Only one handler is allowed for %s of type "%s". However, %d were found: %s', $this->topic, $class, count($refs), implode(', ', $refs)));
                 }
             }
         }
diff --git a/tests/Bridge/Symfony/DependencyInjection/CompilerPass/MessageHandlersLocatorPassTest.php b/tests/Bridge/Symfony/DependencyInjection/CompilerPass/MessageHandlersLocatorPassTest.php
index faf96b7..318bcac 100644
--- a/tests/Bridge/Symfony/DependencyInjection/CompilerPass/MessageHandlersLocatorPassTest.php
+++ b/tests/Bridge/Symfony/DependencyInjection/CompilerPass/MessageHandlersLocatorPassTest.php
@@ -52,7 +52,7 @@ public function testMultipleMessageHandlingProcess(): void
     public function testInvalidSingleMessageHandlingProcess(): void
     {
         $this->expectException(LogicException::class);
-        $this->expectExceptionMessage('Only one handler is allowed for message of type "OpenSolid\Tests\Bus\Fixtures\MyMessage".');
+        $this->expectExceptionMessage('Only one handler is allowed for message of type "OpenSolid\Tests\Bus\Fixtures\MyMessage". However, 2 were found: handler_1, handler_2');
 
         $container = new ContainerBuilder();
         $this->configureContainer($container, allowMultiple: false);