diff --git a/API.php b/API.php
index 6b3ee5b..6817252 100755
--- a/API.php
+++ b/API.php
@@ -92,11 +92,12 @@ public function getValuesForAlertInPast($idAlert, $subPeriodN)
      */
 	public function getAlerts($idSites, $ifSuperUserReturnAllAlerts = false)
 	{
+        $idSites = Site::getIdSitesFromIdSitesString($idSites);
+
         if (empty($idSites)) {
             return array();
         }
 
-        $idSites = Site::getIdSitesFromIdSitesString($idSites);
         Piwik::checkUserHasViewAccess($idSites);
 
         if (Piwik::hasUserSuperUserAccess() && $ifSuperUserReturnAllAlerts) {