diff --git a/framework/helpers/BaseArrayHelper.php b/framework/helpers/BaseArrayHelper.php
index b866679..c79b441 100644
--- a/framework/helpers/BaseArrayHelper.php
+++ b/framework/helpers/BaseArrayHelper.php
@@ -467,15 +467,15 @@ class BaseArrayHelper
     public static function htmlEncode($data, $valuesOnly = true, $charset = null)
     {
         if ($charset === null) {
-            $charset = Yii::$app->charset;
+            $charset = Yii::$app ? Yii::$app->charset : 'UTF-8';
         }
         $d = [];
         foreach ($data as $key => $value) {
             if (!$valuesOnly && is_string($key)) {
-                $key = htmlspecialchars($key, ENT_QUOTES, $charset);
+                $key = htmlspecialchars($key, ENT_QUOTES | ENT_SUBSTITUTE, $charset);
             }
             if (is_string($value)) {
-                $d[$key] = htmlspecialchars($value, ENT_QUOTES, $charset);
+                $d[$key] = htmlspecialchars($value, ENT_QUOTES | ENT_SUBSTITUTE, $charset);
             } elseif (is_array($value)) {
                 $d[$key] = static::htmlEncode($value, $valuesOnly, $charset);
             } else {
diff --git a/tests/framework/helpers/ArrayHelperTest.php b/tests/framework/helpers/ArrayHelperTest.php
index ed2de00..fcc3452 100644
--- a/tests/framework/helpers/ArrayHelperTest.php
+++ b/tests/framework/helpers/ArrayHelperTest.php
@@ -443,7 +443,8 @@ class ArrayHelperTest extends TestCase
             [
                 '<>' => 'a<>b',
                 '23' => true,
-            ]
+            ],
+            'invalid' => "a\x80b",
         ];
         $this->assertEquals([
             'abc' => '123',
@@ -453,7 +454,8 @@ class ArrayHelperTest extends TestCase
             [
                 '<>' => 'a&lt;&gt;b',
                 '23' => true,
-            ]
+            ],
+            'invalid' => 'a�b',
         ], ArrayHelper::htmlEncode($array));
         $this->assertEquals([
             'abc' => '123',
@@ -463,7 +465,8 @@ class ArrayHelperTest extends TestCase
             [
                 '&lt;&gt;' => 'a&lt;&gt;b',
                 '23' => true,
-            ]
+            ],
+            'invalid' => 'a�b',
         ], ArrayHelper::htmlEncode($array, false));
     }