diff --git a/src/Phois/Whois/Whois.php b/src/Phois/Whois/Whois.php
index 2740c4f..e98852a 100644
--- a/src/Phois/Whois/Whois.php
+++ b/src/Phois/Whois/Whois.php
@@ -17,7 +17,7 @@ class Whois
      */
     public function __construct($domain)
     {
-        $this->domain = $domain;
+        $this->domain = strtolower($domain);
         // check $domain syntax and split full domain name on subdomain and TLDs
         if (
             preg_match('/^([\p{L}\d\-]+)\.((?:[\p{L}\-]+\.?)+)$/ui', $this->domain, $matches)