diff --git a/core/processor.py b/core/processor.py
index b0e9aa86cef18894747f23a95276cd486c30e276..bbb8a445d78a807afe3f47ffe34a37d803ce6fab 100644
--- a/core/processor.py
+++ b/core/processor.py
@@ -114,7 +114,7 @@ class PostProcessor:
         with h5py.File(self._filename, "r") as f:
             prev_idchanges = self._signal.get_id_changes()
 
-        changes_history = list(prev_idchanges) + merge_events
+        changes_history = list(prev_idchanges) + [np.array(x) for x in merge_events]
         self._writer.write("id_changes", data=changes_history)
         # self._writer.write(
         #     "/postprocessing/merge_events/",