diff --git a/Readme.md b/Readme.md
index acc6adb..4e7a685 100644
--- a/Readme.md
+++ b/Readme.md
@@ -13,6 +13,9 @@ This feature is enabled only for admins.
 
 ![Show fieldname](/Resources/Public/Documentation/fieldname.png)
 
+This feature is implemented in the TYPO3 Core since version 9.1 ([Changelog](https://docs.typo3.org/c/typo3/cms-core/master/en-us/Changelog/9.1/Feature-83461-ShowFieldnameNextToTitleInDebugMode.html)). To activate it, you have to set 
+`$GLOBALS['TYPO3_CONF_VARS']['BE']['debug'] = true;`.
+
 ## Installation
 
 Use composer with `composer require georgringer/backend-debug`.
diff --git a/composer.json b/composer.json
index 18fbb3d..7ff5b35 100644
--- a/composer.json
+++ b/composer.json
@@ -20,7 +20,7 @@
 		"GPL-2.0+"
 	],
 	"require": {
-		"typo3/cms-core": "^8.7 || ^9.0"
+		"typo3/cms-core": "^8.7 || ^9.0, <9.1"
 	},
 	"autoload": {
 		"psr-4": {