diff --git a/motmetrics/mot.py b/motmetrics/mot.py
index 4ffc3ad..4eeed9b 100644
--- a/motmetrics/mot.py
+++ b/motmetrics/mot.py
@@ -456,7 +456,7 @@ def merge_event_dataframes(dfs, update_frame_indices=True, update_oids=True, upd
                 copy['HId'] = copy['HId'].map(lambda x: hid_map[x], na_action='ignore')
                 infos['hid_map'] = hid_map
 
-            r = r.append(copy)
+            r = pd.concat([r,copy])
             mapping_infos.append(infos)
 
         if return_mappings: