diff --git a/lib/Horde/Imap/Client/Url.php b/lib/Horde/Imap/Client/Url.php
index d0205275..d0c1ea53 100644
--- a/lib/Horde/Imap/Client/Url.php
+++ b/lib/Horde/Imap/Client/Url.php
@@ -174,7 +174,7 @@ public function __toString()
         $url .= '/';
 
         if (is_null($this->protocol) || ($this->protocol == 'imap')) {
-            $url .= rawurlencode($this->mailbox);
+            $url .= rawurlencode($this->mailbox ?? '');
 
             if (!empty($this->uidvalidity)) {
                 $url .= ';UIDVALIDITY=' . $this->uidvalidity;