diff --git a/libfilemanager.py b/libfilemanager.py
index c0f7aeb..ae196ae 100644
--- a/libfilemanager.py
+++ b/libfilemanager.py
@@ -130,7 +130,7 @@ class FileManager:
                         if self.checkduplicates(filename):
                             duplicates.append(filename)
 
-        return duplicates.sort()
+        return duplicates.sorted()
 
     @staticmethod
     def checkduplicates(filename):