diff --git a/filters/enrich-via-doi.py b/filters/enrich-via-doi.py
index 30403ce..43b529c 100644
--- a/filters/enrich-via-doi.py
+++ b/filters/enrich-via-doi.py
@@ -208,8 +208,7 @@ def publishing_process(d: dict) -> dict[str, str] | None:
     if (pubdate := csl_publish_date(d)) is not None:
         has_detail = True
         res["at_time"] = pubdate
-
-    if (issn := d["ISSN"]) is not None:
+    if (issn := d.get("ISSN", None)) is not None:
         has_detail = True
         # there can be more than one (e.g. different for print / online)
         # if that's the case, use the 1st - we have no more data at hand