diff --git a/model/MailboxHandler.php b/model/MailboxHandler.php
index 99c3e736..03fd85ff 100644
--- a/model/MailboxHandler.php
+++ b/model/MailboxHandler.php
@@ -217,7 +217,7 @@ class MailboxHandler extends PFAHandler {
 
     protected function beforestore() {
 
-        if (isset($this->values['quota']) && $this->values['quota'] != -1) {
+        if (isset($this->values['quota']) && is_numeric($this->values['quota']) && $this->values['quota'] != -1) {
             $multiplier = Config::read('quota_multiplier');
             if ($multiplier == 0 || !is_numeric($multiplier)) { // or empty string, or null, or false...
                 $multiplier = 1;