diff --git a/nomenclature/codelist.py b/nomenclature/codelist.py
index 373f7068..af4ff77b 100644
--- a/nomenclature/codelist.py
+++ b/nomenclature/codelist.py
@@ -357,7 +357,7 @@ def _check_string(attr, value):
                     _check_string(attr, item)
 
         for code in self.mapping.values():
-            if not code.repository:
+            if not code.from_external_repository:
                 for attr, value in code.model_dump(exclude="file").items():
                     _check_string(attr, value)
         if errors: