diff --git a/lib/Service/DiffTaskService.php b/lib/Service/DiffTaskService.php
index 0f53d36..1ff8fad 100644
--- a/lib/Service/DiffTaskService.php
+++ b/lib/Service/DiffTaskService.php
@@ -53,7 +53,7 @@ class DiffTaskService {
         }
     }
 
-    function create(int $groupfolderId, string $snapshotId, string $userId, $progressCallback): DiffTask {
+    function create(int $groupfolderId, string $snapshotId, string $userId, Callable $progressCallback = null): DiffTask {
         $snapshotPath = $this->pathManager->getGroupFolderSnapshotDirectory($groupfolderId, $snapshotId);
         $groupfolderPath = $this->pathManager->getGroupFolderDirectory($groupfolderId);
 
@@ -83,7 +83,7 @@ class DiffTaskService {
                 $newResult = $this->diffTaskResultMapper->insert($newResult);
             },
             function($numDoneFiles) use ($progressCallback, &$numFiles) {
-                if(($numFiles != 0) && ($numFiles != $numDoneFiles)) {
+                if(isset($progressCallback) && ($numFiles != 0) && ($numFiles != $numDoneFiles)) {
                     ($progressCallback)([
                         "overallFiles" => $numFiles,
                         "doneFiles" => $numDoneFiles,
@@ -98,14 +98,15 @@ class DiffTaskService {
 
         $diffTask->diff();
 
-        ($progressCallback)([
-            "overallFiles" => $numFiles,
-            "doneFiles" => $numFiles,
-            "progress" => 1.0,
-            "progressPercent" => "100.00%",
-            // TODO: include task results in object
-            "result" => $task,
-        ]);
+        if(isset($progressCallback)) {
+            ($progressCallback)([
+                "overallFiles" => $numFiles,
+                "doneFiles" => $numFiles,
+                "progress" => 1.0,
+                "progressPercent" => "100.00%",
+                "result" => $task,
+            ]);
+        }
 
         return $task;
     }