diff --git a/framework/yii/YiiBase.php b/framework/yii/YiiBase.php
index c96a969..f04903d 100644
--- a/framework/yii/YiiBase.php
+++ b/framework/yii/YiiBase.php
@@ -335,7 +335,7 @@ class YiiBase
 
 		include($classFile);
 
-		if (!class_exists($className, false) && !interface_exists($className, false) &&
+		if (YII_DEBUG && !class_exists($className, false) && !interface_exists($className, false) &&
 			(!function_exists('trait_exists') || !trait_exists($className, false))) {
 			throw new UnknownClassException("Unable to find '$className' in file: $classFile");
 		}