diff --git a/framework/helpers/BaseUrl.php b/framework/helpers/BaseUrl.php
index 69b240456a6..5623aec5128 100644
--- a/framework/helpers/BaseUrl.php
+++ b/framework/helpers/BaseUrl.php
@@ -223,7 +223,7 @@ public static function to($url = '', $scheme = false)
             return $url;
         }
 
-        if (($pos = strpos($url, ':')) === false || !ctype_alpha(substr($url, 0, $pos))) {
+        if (static::isRelative($url)) {
             // turn relative URL into absolute
             $url = static::getUrlManager()->getHostInfo() . '/' . ltrim($url, '/');
         }
diff --git a/tests/framework/helpers/UrlTest.php b/tests/framework/helpers/UrlTest.php
index 391d894c737..1be2ffe43e9 100644
--- a/tests/framework/helpers/UrlTest.php
+++ b/tests/framework/helpers/UrlTest.php
@@ -186,6 +186,13 @@ public function testTo()
         $this->assertEquals('https://example.com/#test', Url::to('@web5', 'https'));
         $this->assertEquals('//example.com/#test', Url::to('@web5', ''));
 
+        // @see https://github.com/yiisoft/yii2/issues/13156
+        \Yii::setAlias('@cdn', '//cdn.example.com');
+        $this->assertEquals('http://cdn.example.com/images/logo.gif', Url::to('@cdn/images/logo.gif', 'http'));
+        $this->assertEquals('//cdn.example.com/images/logo.gif', Url::to('@cdn/images/logo.gif', ''));
+        $this->assertEquals('https://cdn.example.com/images/logo.gif', Url::to('@cdn/images/logo.gif', 'https'));
+        \Yii::setAlias('@cdn', null);
+
         //In case there is no controller, throw an exception
         $this->removeMockedAction();