diff --git a/gap/structure.gi b/gap/structure.gi
index 3b077ab..3cb9ba6 100644
--- a/gap/structure.gi
+++ b/gap/structure.gi
@@ -341,7 +341,7 @@ InstallMethod(OppositeFamily, "for a family",
     local F, elt, elt_props, opp_props, prop, filt, opp_filt;
     elt := Representative(fam);
     elt_props := CategoriesOfObject(elt);
-    Append(elt_props(KnownTruePropertiesOfObject(elt)));
+    Append(elt_props, KnownTruePropertiesOfObject(elt));
     opp_props := [];
     for prop in elt_props do
       if KnowsDictionary(OFDir@, prop) then