diff --git a/libfilemanager.py b/libfilemanager.py
index ae196ae..db51349 100644
--- a/libfilemanager.py
+++ b/libfilemanager.py
@@ -130,7 +130,7 @@ class FileManager:
                         if self.checkduplicates(filename):
                             duplicates.append(filename)
 
-        return duplicates.sorted()
+        return sorted(duplicates)
 
     @staticmethod
     def checkduplicates(filename):