diff --git a/swh/core/db/tests/db_testing.py b/swh/core/db/tests/db_testing.py
--- a/swh/core/db/tests/db_testing.py
+++ b/swh/core/db/tests/db_testing.py
@@ -293,7 +293,9 @@
         cls.dbname = cls.TEST_DB_NAME  # XXX to kill?
 
         dump_files = cls.TEST_DB_DUMP
-        if isinstance(dump_files, str):
+        if dump_files is None:
+            dump_files = []
+        elif isinstance(dump_files, str):
             dump_files = [dump_files]
         all_dump_files = []
         for files in dump_files: