diff --git a/gap/oper.gi b/gap/oper.gi
index fe49349e7..64a8f981c 100644
--- a/gap/oper.gi
+++ b/gap/oper.gi
@@ -2064,7 +2064,7 @@ InstallMethod(IsOrderIdeal, "for a digraph and a list of vertices",
 function(D, roots)
   local reachable_vertices, vertex_in_subset, i, N;
   if not IsPartialOrderDigraph(D) then
-    ErrorNoReturn("the 1st element (a digraph) must be a partial order digraph");
+    ErrorNoReturn("the 1st argument (a digraph) must be a partial order digraph");
   fi;
 
   N := Length(roots);