diff --git a/lib/Service/Attachment/AttachmentService.php b/lib/Service/Attachment/AttachmentService.php
index 6bc208de3e..64eab72a79 100644
--- a/lib/Service/Attachment/AttachmentService.php
+++ b/lib/Service/Attachment/AttachmentService.php
@@ -82,6 +82,10 @@ public function addFile(string $userId, UploadedFile $file): LocalAttachment {
 		$attachment->setMimeType($file->getMimeType());
 
 		$persisted = $this->mapper->insert($attachment);
+		if ($persisted->id === null) {
+			throw new UploadException('Given attachmentId is null');
+		}
+
 		try {
 			$this->storage->save($userId, $persisted->id, $file);
 		} catch (UploadException $ex) {
@@ -100,6 +104,10 @@ public function addFileFromString(string $userId, string $name, string $mime, st
 		$attachment->setMimeType($mime);
 
 		$persisted = $this->mapper->insert($attachment);
+		if ($persisted->id === null) {
+			throw new UploadException('Given attachmentId is null');
+		}
+
 		try {
 			$this->storage->saveContent($userId, $persisted->id, $fileContents);
 		} catch (NotFoundException|NotPermittedException $e) {
diff --git a/lib/Service/Attachment/AttachmentStorage.php b/lib/Service/Attachment/AttachmentStorage.php
index 94853dd1cf..b5eb6acab3 100644
--- a/lib/Service/Attachment/AttachmentStorage.php
+++ b/lib/Service/Attachment/AttachmentStorage.php
@@ -48,14 +48,14 @@ private function getAttachmentFolder($userId): ISimpleFolder {
 	 * Copy uploaded file content to a app data file
 	 *
 	 * @param string $userId
-	 * @param int $attachmentId
+	 * @param int|string $attachmentId
 	 * @param UploadedFile $uploadedFile
 	 *
 	 * @throws UploadException
 	 *
 	 * @return void
 	 */
-	public function save(string $userId, int $attachmentId, UploadedFile $uploadedFile): void {
+	public function save(string $userId, int|string $attachmentId, UploadedFile $uploadedFile): void {
 		$folder = $this->getAttachmentFolder($userId);
 
 		$file = $folder->newFile((string)$attachmentId);
@@ -80,12 +80,12 @@ public function save(string $userId, int $attachmentId, UploadedFile $uploadedFi
 	 * Copy uploaded file content to a app data file
 	 *
 	 * @param string $userId
-	 * @param int $attachmentId
+	 * @param int|string $attachmentId
 	 *
 	 * @return void
 	 * @throws NotFoundException|NotPermittedException
 	 */
-	public function saveContent(string $userId, int $attachmentId, string $fileContent): void {
+	public function saveContent(string $userId, int|string $attachmentId, string $fileContent): void {
 		$folder = $this->getAttachmentFolder($userId);
 		$file = $folder->newFile((string)$attachmentId);
 		$file->putContent($fileContent);