diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index 7cde8c6278..fd522480e6 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -61,6 +61,7 @@ Yii Framework 2 Change Log
 - Bug #20608: Fix `@return` annotations for `yii\rest\Serializer` methods (mspirkov)
 - Bug #20610: Fix `@var` annotation for `ActiveQueryTrait::$with` (mspirkov)
 - Bug #20611: Fix `@return` annotations for `yii\i18n\GettextMoFile` methods (mspirkov)
+- Bug #20618: Fix `@var` annotation for `yii\web\Response::$acceptMimeType` (mspirkov)
 - Bug #20617: Fix `@return` annotation for `DataColumn::getDataCellValue()` (mspirkov)
 
 
diff --git a/framework/web/Response.php b/framework/web/Response.php
index 9b617c8594..ddfb8c34a1 100644
--- a/framework/web/Response.php
+++ b/framework/web/Response.php
@@ -103,7 +103,7 @@ class Response extends \yii\base\Response
      */
     public $format = self::FORMAT_HTML;
     /**
-     * @var string the MIME type (e.g. `application/json`) from the request ACCEPT header chosen for this response.
+     * @var string|null the MIME type (e.g. `application/json`) from the request ACCEPT header chosen for this response.
      * This property is mainly set by [[\yii\filters\ContentNegotiator]].
      */
     public $acceptMimeType;