diff --git a/src/TelegramHandler.php b/src/TelegramHandler.php
index 5301a34..44df055 100644
--- a/src/TelegramHandler.php
+++ b/src/TelegramHandler.php
@@ -150,7 +150,7 @@ private function sendMessage(string $text): void
      * @param string $defaultConfigKey
      * @return string
      */
-    private function getConfigValue($key, $defaultConfigKey = null): string
+    private function getConfigValue($key, $defaultConfigKey = null): ?string
     {
         if (isset($this->config[$key])) {
             return $this->config[$key];