diff --git a/framework/UPGRADE.md b/framework/UPGRADE.md
index 5d9e903e4d..ec571a5077 100644
--- a/framework/UPGRADE.md
+++ b/framework/UPGRADE.md
@@ -96,6 +96,16 @@ Upgrade from Yii 2.0.35
       // $current === 'second' (on second iteration)
   }
   ```
+  
+* `$this` in an inline validator defined as closure now refers to model instance. If you need to access the object registering
+  the validator, pass its instance through use statement:
+  
+  ```php
+  $registrar = $this;
+  $validator = function($attribute, $params, $validator, $current) use ($registrar) {
+      // ...
+  }
+  ```
 
 * If you have any controllers that override the `init()` method, make sure they are calling `parent::init()` at
   the beginning, as demonstrated in the [component guide](https://www.yiiframework.com/doc/guide/2.0/en/concept-components).