diff --git a/source/patch_track.py b/source/patch_track.py
index 401411d..81fb0e2 100644
--- a/source/patch_track.py
+++ b/source/patch_track.py
@@ -77,7 +77,7 @@ def patch_track(self, tracks, total_track="?"):
     def clean_process():
         nonlocal error_count, error_max, process_list
 
-        for track_file, process in process_list.items():
+        for track_file, process in process_list.copy().items():
             if process is not None:
                 if process.poll() is None:
                     pass  # if the process is still running