diff --git a/swh/indexer/metadata_dictionary/npm.py b/swh/indexer/metadata_dictionary/npm.py
--- a/swh/indexer/metadata_dictionary/npm.py
+++ b/swh/indexer/metadata_dictionary/npm.py
@@ -276,6 +276,10 @@
                 # does not allow accessing that from metadata mappings.
                 # (Plus, an hypothetical license mapping would eventually pick it up)
                 return
+            if " " in s:
+                # Either an SPDX expression, or unusable data
+                # TODO: handle it
+                return
             return SPDX + s
 
     def normalize_keywords(self, lst):