diff --git a/lib/ext/Syncroton/Command/Sync.php b/lib/ext/Syncroton/Command/Sync.php
--- a/lib/ext/Syncroton/Command/Sync.php
+++ b/lib/ext/Syncroton/Command/Sync.php
@@ -560,6 +560,11 @@
                 if ($this->_logger instanceof Zend_Log)
                     $this->_logger->warn(__METHOD__ . '::' . __LINE__ . " Detected an unknown folder, skipping {$collectionData->collectionId}.");
 
+                $collection = $collections->appendChild($this->_outputDom->createElementNS('uri:AirSync', 'Collection'));
+                $collection->appendChild($this->_outputDom->createElementNS('uri:AirSync', 'SyncKey', $collectionData->syncKey));
+                $collection->appendChild($this->_outputDom->createElementNS('uri:AirSync', 'CollectionId', $collectionData->collectionId));
+                $collection->appendChild($this->_outputDom->createElementNS('uri:AirSync', 'Status', self::STATUS_OBJECT_NOT_FOUND));
+
                 continue;
             }