diff --git a/src/Auth/Process/EntityCategory.php b/src/Auth/Process/EntityCategory.php
index dcf96b4..82d8759 100644
--- a/src/Auth/Process/EntityCategory.php
+++ b/src/Auth/Process/EntityCategory.php
@@ -104,7 +104,7 @@ public function __construct(array $config, $reserved)
 
             Assert::string(
                 $index,
-                "Unspecified allowed attributes for the '$value' category.",
+                "Identifier of a category must be a string. '$index' is set.",
                 Error\ConfigurationError::class,
             );