diff --git a/program/lib/Roundcube/rcube_imap_cache.php b/program/lib/Roundcube/rcube_imap_cache.php
index a99f554a2d8..40aa061eb6a 100644
--- a/program/lib/Roundcube/rcube_imap_cache.php
+++ b/program/lib/Roundcube/rcube_imap_cache.php
@@ -1094,7 +1094,7 @@ public function synchronize($mailbox)
                 $mbox_data = $this->imap->folder_data($mailbox);
 
                 // Removed messages found
-                $uids = isset($mbox_data['VANISHED']) ? rcube_imap_generic::uncompressMessageSet($mbox_data['VANISHED']) : null;
+                $uids = !empty($mbox_data['VANISHED']) ? rcube_imap_generic::uncompressMessageSet($mbox_data['VANISHED']) : null;
                 if (!empty($uids)) {
                     $removed = array_merge($removed, $uids);
                     // Invalidate index
diff --git a/program/lib/Roundcube/rcube_imap_generic.php b/program/lib/Roundcube/rcube_imap_generic.php
index fc2b7585daf..ecb1f2f12e7 100644
--- a/program/lib/Roundcube/rcube_imap_generic.php
+++ b/program/lib/Roundcube/rcube_imap_generic.php
@@ -2062,7 +2062,8 @@ public function fetchHeaderIndex($mailbox, $message_set, $index_field = '', $ski
     {
         // Validate input
         if (is_array($message_set)) {
-            if (($message_set = $this->compressMessageSet($message_set)) === 'INVALID') {
+            $message_set = $this->compressMessageSet($message_set);
+            if ($message_set === '' || $message_set === 'INVALID') {
                 return false;
             }
         } elseif (empty($message_set)) {
@@ -4042,7 +4043,7 @@ protected static function r_implode($element)
      * @param array|string $messages Message identifiers
      * @param bool         $force    Forces compression of any size
      *
-     * @return string Compressed sequence-set or 'INVALID' on invalid or empty input
+     * @return string Compressed sequence-set or 'INVALID' on invalid input
      */
     public static function compressMessageSet($messages, $force = false)
     {
@@ -4064,7 +4065,7 @@ public static function compressMessageSet($messages, $force = false)
         }
 
         if (empty($messages)) {
-            return 'INVALID';
+            return '';
         }
 
         sort($messages);