diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index 073a3ce..a1d7bdf 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -9,6 +9,7 @@ Yii Framework 2 Change Log
 - Bug #1500: Log messages exported to files are not separated by newlines (omnilight, qiangxue)
 - Bug #1509: The SQL for creating Postgres RBAC tables is incorrect (qiangxue)
 - Bug #1545: It was not possible to execute db Query twice, params where missing (cebe)
+- Bug #1591: StringValidator is accessing undefined property (qiangxue)
 - Bug: Fixed `Call to a member function registerAssetFiles() on a non-object` in case of wrong `sourcePath` for an asset bundle (samdark)
 - Bug: Fixed incorrect event name for `yii\jui\Spinner` (samdark)
 - Bug: Json::encode() did not handle objects that implement JsonSerializable interface correctly (cebe)
diff --git a/framework/yii/validators/StringValidator.php b/framework/yii/validators/StringValidator.php
index a93fb72..dbc4001 100644
--- a/framework/yii/validators/StringValidator.php
+++ b/framework/yii/validators/StringValidator.php
@@ -174,7 +174,7 @@ class StringValidator extends Validator
 			$options['is'] = $this->length;
 			$options['notEqual'] = Html::encode(strtr($this->notEqual, [
 				'{attribute}' => $label,
-				'{length}' => $this->is,
+				'{length}' => $this->length,
 			]));
 		}
 		if ($this->skipOnEmpty) {