diff --git a/publish/src/rdf/materialize.rs b/publish/src/rdf/materialize.rs
index 14b2256..c859cf8 100644
--- a/publish/src/rdf/materialize.rs
+++ b/publish/src/rdf/materialize.rs
@@ -100,8 +100,8 @@ pub fn super_classes(store: &mut Store) -> error::Result<()> {
         ?item a ?parent .
     }
 } WHERE {
-    GRAPH ?g1 { ?item a ?class }
-    GRAPH ?g2 { ?class rdfs:subClassOf+ ?parent }
+    GRAPH ?g1 { ?class rdfs:subClassOf+ ?parent }
+    OPTIONAL { GRAPH ?g2 { ?item a ?class } }
 }"#)?;
 
     let old_size = store.len()?;