diff --git a/askomics/libaskomics/TriplestoreExplorer.py b/askomics/libaskomics/TriplestoreExplorer.py
index 5f926522..c5c8a769 100644
--- a/askomics/libaskomics/TriplestoreExplorer.py
+++ b/askomics/libaskomics/TriplestoreExplorer.py
@@ -526,7 +526,7 @@ def get_abstraction_relations(self):
                 ?node rdfs:range ?range_uri .
                 ?node askomics:uri ?property_uri
                 # Retrocompatibility
-                OPTIONAL {?node askomics:uri ?property_uri}
+                OPTIONAL {{?node askomics:uri ?property_uri}}
             }}
             # Relation of entity (or motherclass of entity)
             {{