diff --git a/lib/Cake/Model/Datasource/CakeSession.php b/lib/Cake/Model/Datasource/CakeSession.php
index 695bd2ef6..96a66d899 100644
--- a/lib/Cake/Model/Datasource/CakeSession.php
+++ b/lib/Cake/Model/Datasource/CakeSession.php
@@ -348,7 +348,7 @@ class CakeSession {
 		$config = self::read('Config');
 		$validAgent = (
 			Configure::read('Session.checkAgent') === false ||
-			self::$_userAgent == $config['userAgent']
+			isset($config['userAgent']) && self::$_userAgent === $config['userAgent']
 		);
 		return ($validAgent && self::$time <= $config['time']);
 	}