diff --git a/parse_peak_so.py b/parse_peak_so.py
index 00e5e7d0d3ccde0175dbc1040be4619b41b6e607..1480644a83b24700778665a06b78c3147416052c 100644
--- a/parse_peak_so.py
+++ b/parse_peak_so.py
@@ -208,7 +208,7 @@ def convert_frame_to_so(lines, library_map_path, worker_number):
                 frame_to_so[frame] = so_path
                 last_frame_row.append(so_path)
                 break
-            first_frame_rows.append(last_frame_row)
+            first_frame_rows.append(tuple(last_frame_row))
     return first_frame_rows