diff --git a/src/Symfony/Component/Notifier/Bridge/Slack/Block/SlackHeaderBlock.php b/src/Symfony/Component/Notifier/Bridge/Slack/Block/SlackHeaderBlock.php
index 5a4d4189ca5b1..6595b78b5a6c2 100644
--- a/src/Symfony/Component/Notifier/Bridge/Slack/Block/SlackHeaderBlock.php
+++ b/src/Symfony/Component/Notifier/Bridge/Slack/Block/SlackHeaderBlock.php
@@ -25,7 +25,7 @@ final class SlackHeaderBlock extends AbstractSlackBlock
 
     public function __construct(string $text)
     {
-        if (\strlen($text) > self::TEXT_LIMIT) {
+        if (mb_strlen($text, 'UTF-8') > self::TEXT_LIMIT) {
             throw new LengthException(sprintf('Maximum length for the text is %d characters.', self::TEXT_LIMIT));
         }
 
diff --git a/src/Symfony/Component/Notifier/Bridge/Slack/composer.json b/src/Symfony/Component/Notifier/Bridge/Slack/composer.json
index 2a6ff04f45b08..d0753053ff44a 100644
--- a/src/Symfony/Component/Notifier/Bridge/Slack/composer.json
+++ b/src/Symfony/Component/Notifier/Bridge/Slack/composer.json
@@ -19,7 +19,8 @@
         "php": ">=7.2.5",
         "symfony/deprecation-contracts": "^2.1",
         "symfony/http-client": "^4.3|^5.0",
-        "symfony/notifier": "~5.3.0"
+        "symfony/notifier": "~5.3.0",
+        "symfony/polyfill-mbstring": "^1.0"
     },
     "require-dev": {
         "symfony/event-dispatcher": "^4.3|^5.0"