diff --git a/src/index_notation/index_notation.cpp b/src/index_notation/index_notation.cpp
index 1e462e47a..f1f2af195 100644
--- a/src/index_notation/index_notation.cpp
+++ b/src/index_notation/index_notation.cpp
@@ -198,7 +198,7 @@ struct Isomorphic : public IndexNotationVisitorStrict {
       return;
     }
     for (auto aset = anode->indexSetModes.begin(), bset = bnode->indexSetModes.begin(); aset != anode->indexSetModes.end(); ++aset, ++bset) {
-      if (aset->first != bset->first || *aset->second.set != *bset->second.set) {
+      if (aset->first != bset->first || aset->second.set->size() != bset->second.set->size()) {
         eq = false;
         return;
       }