diff --git a/src/Zendesk/API/Utilities/OAuth.php b/src/Zendesk/API/Utilities/OAuth.php
index 45a83052..32d782c4 100644
--- a/src/Zendesk/API/Utilities/OAuth.php
+++ b/src/Zendesk/API/Utilities/OAuth.php
@@ -83,7 +83,7 @@ public static function getAuthUrl($subdomain, array $options, $domain = 'zendesk
      */
     private static function validateSubdomain($subdomain)
     {
-        if (! preg_match('/^[0-9A-Za-z-]+$/', $subdomain)) {
+        if (! preg_match('/^[A-Za-z0-9](?:[A-Za-z0-9\-]{0,61}[A-Za-z0-9])?$/', $subdomain)) {
             throw new InvalidArgumentException('Invalid Zendesk subdomain.');
         }
     }