diff --git a/lib/Service/FilesService.php b/lib/Service/FilesService.php
index bb26aa8..47cda38 100644
--- a/lib/Service/FilesService.php
+++ b/lib/Service/FilesService.php
@@ -473,7 +473,7 @@ private function generateFilesDocumentFromFile(string $viewerId, Node $file): Fi
 			throw new FileIsNotIndexableException();
 		}
 
-		if ($file->getMimetype() !== null) {
+		if (is_string($file->getMimetype())) {
 			$document->setMimetype($file->getMimetype());
 		}