diff --git a/src/Cache/GeoIpCacheWarmer.php b/src/Cache/GeoIpCacheWarmer.php
index 438ae56..7d67b38 100644
--- a/src/Cache/GeoIpCacheWarmer.php
+++ b/src/Cache/GeoIpCacheWarmer.php
@@ -78,9 +78,9 @@ public function isOptional()
     /**
      * Warms up the cache.
      *
-     * @param string $cacheDir The cache directory
+     * {@inheritdoc}
      */
-    public function warmUp($cacheDir)
+    public function warmUp($cacheDir): void
     {
         //avoid downloading if db is up to date
         if ($this->isGeoDatabaseUpToDate()) {