diff --git a/src/DI/McpExtension.php b/src/DI/McpExtension.php
index 7cb052d..ff7f127 100644
--- a/src/DI/McpExtension.php
+++ b/src/DI/McpExtension.php
@@ -43,7 +43,7 @@ public function getConfigSchema(): Schema
 		$parameters = $this->getContainerBuilder()->parameters;
 
 		$expectService = Expect::anyOf(
-			Expect::string()->required()->assert(static fn ($input) => str_starts_with($input, '@') || class_exists($input) || interface_exists($input)),
+			Expect::string()->required()->assert(static fn ($input): bool => is_string($input) && (str_starts_with($input, '@') || class_exists($input) || interface_exists($input))),
 			Expect::type(Statement::class)->required(),
 		);