diff --git a/personal_data/__main__.py b/personal_data/__main__.py
index b10424c..2edcb50 100644
--- a/personal_data/__main__.py
+++ b/personal_data/__main__.py
@@ -7,7 +7,9 @@ from personal_data.notification import NotificationType
 
 def parse_arguments():
     available_scraper_names = personal_data.main.available_scraper_names()
-    parser = argparse.ArgumentParser()
+    parser = argparse.ArgumentParser(
+        epilog='Available fetchers: ' + ' '.join(available_scraper_names),
+    )
     parser.add_argument(
         'fetchers',
         metavar='FETCHER',
diff --git a/personal_data/main.py b/personal_data/main.py
index c427358..10a016d 100644
--- a/personal_data/main.py
+++ b/personal_data/main.py
@@ -80,7 +80,7 @@ def equals_without_fields(
 
 def deduplicate_by_ignoring_certain_fields(
     dicts: list[dict],
-    deduplicate_ignore_columns: list[str],
+    deduplicate_ignore_columns: Iterable[str],
 ) -> list[dict]:
     """Removes duplicates that occur when ignoring certain columns.
 
@@ -104,6 +104,8 @@ def deduplicate_dicts(
     deduplicate_mode: personal_data.data.DeduplicateMode,
     deduplicate_ignore_columns: list[str],
 ) -> tuple[Sequence[dict], list[str]]:
+    assert isinstance(deduplicate_ignore_columns, list), deduplicate_ignore_columns
+
     fieldnames = []
     for d in dicts:
         for k in d.keys():
@@ -143,6 +145,8 @@ def extend_csv_file(
     deduplicate_mode: personal_data.data.DeduplicateMode,
     deduplicate_ignore_columns: list[str],
 ) -> dict:
+    assert isinstance(deduplicate_ignore_columns, list), deduplicate_ignore_columns
+
     dicts = []
     try:
         with open(filename) as csvfile:
@@ -292,7 +296,7 @@ def main(
             f'output/{scraper.dataset_name}.csv',
             result_rows,
             deduplicate_mode=scraper.deduplicate_mode,
-            deduplicate_ignore_columns=scraper.deduplicate_ignore_columns,
+            deduplicate_ignore_columns=scraper.deduplicate_ignore_columns(),
         )
         logger.info('Scraper done: %s', scraper.dataset_name)