diff --git a/src/sparqlunicorn_ontdoc/docgeneration.py b/src/sparqlunicorn_ontdoc/docgeneration.py
index b530222d..f4ed2ce8 100644
--- a/src/sparqlunicorn_ontdoc/docgeneration.py
+++ b/src/sparqlunicorn_ontdoc/docgeneration.py
@@ -650,7 +650,10 @@ def getClassTree(self,graph, uritolabel,classidset,uritotreeitem):
         for res in results:
             #print(res)
             if "_:" not in str(res["subject"]) and str(res["subject"]).startswith("http"):
-                ress[str(res["subject"])] = {"super": res["supertype"], "label": res["label"]}
+                if "_:" not in str(res["supertype"]) and str(res["supertype"]).startswith("http"):
+                    ress[str(res["subject"])] = {"super": res["supertype"], "label": res["label"]}
+                else:
+                    ress[str(res["subject"])] = {"super": None, "label": res["label"]}
         #print(ress)
         for cls in ress:
             for obj in graph.subjects(URIRef(self.typeproperty), URIRef(cls),True):