diff --git a/csd3-side/scripts/lsst-backup.py b/csd3-side/scripts/lsst-backup.py
index 06073ea..74d89a5 100644
--- a/csd3-side/scripts/lsst-backup.py
+++ b/csd3-side/scripts/lsst-backup.py
@@ -1005,6 +1005,7 @@ def process_files(s3_host, access_key, secret_key, bucket_name, current_objects,
             print(f'Loaded collate list from {collate_list_file}, len={len(to_collate)}.', flush=True)
             if not current_objects.empty:
                 # now using pandas for both current_objects and to_collate - this could be re-written to using vectorised operations
+                droplist = []
                 for i in range(len(to_collate['object_names'])):
                     # print(zip_object_names)
                     cmp = [x.replace(destination_dir+'/', '') for x in to_collate.iloc[i]['object_names']]
@@ -1015,10 +1016,11 @@ def process_files(s3_host, access_key, secret_key, bucket_name, current_objects,
                         existing_zip_contents = current_objects[current_objects['METADATA'].isin([cmp])]['METADATA'].values[0]
                         if all([x in existing_zip_contents for x in cmp]):
                             print(f'Zip file {destination_dir}/collated_{i+1}.zip from {collate_list_file} already exists and file lists match - skipping.')
-                            to_collate = to_collate.drop(i)
+                            droplist.append(i)
                             continue
                         else:
                             print(f'Zip file {destination_dir}/collated_{i+1}.zip from {collate_list_file} already exists but file lists do not match - reuploading.')
+                to_collate.drop(droplist, inplace=True)
         if save_collate_file:
             print(f'Saving collate list to {collate_list_file}, len={len(to_collate)}.')
             # with open(collate_list_file, 'w') as f: