diff --git a/gap/congruences/congsemigraph.gd b/gap/congruences/congsemigraph.gd
index b665d5e10..bdaaa081c 100644
--- a/gap/congruences/congsemigraph.gd
+++ b/gap/congruences/congsemigraph.gd
@@ -23,6 +23,6 @@ DeclareOperation("CongruenceByWangPair",
 DeclareOperation("AsCongruenceByWangPair", [IsSemigroupCongruence]);
 
 DeclareOperation("MinimalHereditarySubsetsVertex",
-                 [IsGraphInverseSemigroup, IsPosInt]);
+                 [IsDigraph, IsPosInt]);
 
 DeclareAttribute("GeneratingCongruencesOfLattice", IsGraphInverseSemigroup);