diff --git a/Net/SMTP.php b/Net/SMTP.php
index ab00e4e..a2ccaeb 100644
--- a/Net/SMTP.php
+++ b/Net/SMTP.php
@@ -213,7 +213,7 @@ public function __construct($host = null, $port = null, $localhost = null,
         $this->gssapi_cname     = $gssapi_cname;
 
         /* If PHP krb5 extension is loaded, we enable GSSAPI method. */
-        if (extension_loaded('krb5')) {
+        if (isset($gssapi_principal) && extension_loaded('krb5')) {
             $this->setAuthMethod('GSSAPI', array($this, 'authGSSAPI'));
         }