diff --git a/scripts/clean_spa.py b/scripts/clean_spa.py
index f3d8f7d..1bb6df5 100644
--- a/scripts/clean_spa.py
+++ b/scripts/clean_spa.py
@@ -85,7 +85,7 @@ with jsonlines.open("annas_archive_es.jsonl") as reader:
                 dump = dump.decode() + "\n"
                 if not item["isbn13"]:
                     additem(dump, index, "unregistered")
-                elif item["isbn13"] not in isbn:
+                elif item["isbn13"] and item["isbn13"] not in isbn:
                     isbn.add(item["isbn13"])
                     additem(dump, index, "registered")
         except Exception as err: