diff --git a/model/MailboxHandler.php b/model/MailboxHandler.php
index 5a912f8c..17b9a8d7 100644
--- a/model/MailboxHandler.php
+++ b/model/MailboxHandler.php
@@ -228,7 +228,7 @@ class MailboxHandler extends PFAHandler {
         }
 
         // Avoid trying to store '' in an integer field
-        if ($this->values['quota'] === '') {
+        if (array_key_exists('quota', $this->values) && $this->values['quota'] === '') {
             $this->values['quota'] = 0;
         }