fixes #1811

framework/yii/web/Request.php

@@ -584,7 +584,9 @@ class Request extends \yii\base\Request
 			throw new InvalidConfigException('Unable to determine the path info of the current request.');
 		}
 
-		if ($pathInfo[0] === '/') {
+		if ($pathInfo === '/') {
+			$pathInfo = '';
+		} else if ($pathInfo[0] === '/') {
 			$pathInfo = substr($pathInfo, 1);
 		}