diff --git a/repositories/condition/property_uprn_lookup.py b/repositories/condition/property_uprn_lookup.py
index cf2b2a344..8a01b8c29 100644
--- a/repositories/condition/property_uprn_lookup.py
+++ b/repositories/condition/property_uprn_lookup.py
@@ -25,4 +25,9 @@ class PropertyUprnLookup(UprnLookup):
     def null_uprn_references(self) -> List[str]:
         """Landlord references present in the portfolio whose UPRN is null —
         surfaced so a load can report them (none expected; ADR-0064)."""
-        raise NotImplementedError
+        references = self._reader.references_for_portfolio(self._portfolio_id)
+        return [
+            ref.landlord_property_id
+            for ref in references
+            if ref.landlord_property_id is not None and ref.uprn is None
+        ]