--- a/src/nfagraph/ng_is_equal.h
+++ b/src/nfagraph/ng_is_equal.h
@@ -51,6 +51,9 @@ u64a hash_holder(const NGHolder &g);
     size_t operator()(const std::shared_ptr<const NGHolder> &h) const {
         return hash_holder(*h);
     }
+    size_t operator()(const std::shared_ptr<NGHolder> &h) const {
+        return hash_holder(*h);
+    }
 };

 struct NGHolderEqual {