diff --git a/faiss/clone_index.cpp b/faiss/clone_index.cpp
index bc08283740..5a1e5cfad2 100644
--- a/faiss/clone_index.cpp
+++ b/faiss/clone_index.cpp
@@ -335,9 +335,10 @@ Index* Cloner::clone_Index(const Index* index) {
         IndexNSG* res = clone_IndexNSG(insg);
 
         // copy the dynamic allocated graph
-        auto& new_graph = res->nsg.final_graph;
-        auto& old_graph = insg->nsg.final_graph;
-        new_graph = std::make_shared<nsg::Graph<int>>(*old_graph);
+        if (auto& old_graph = insg->nsg.final_graph) {
+            auto& new_graph = res->nsg.final_graph;
+            new_graph = std::make_shared<nsg::Graph<int>>(*old_graph);
+        }
 
         res->own_fields = true;
         res->storage = clone_Index(insg->storage);
diff --git a/faiss/cppcontrib/factory_tools.cpp b/faiss/cppcontrib/factory_tools.cpp
index d1f283b8ff..46ffada3e8 100644
--- a/faiss/cppcontrib/factory_tools.cpp
+++ b/faiss/cppcontrib/factory_tools.cpp
@@ -8,8 +8,22 @@
 // -*- c++ -*-
 
 #include <faiss/cppcontrib/factory_tools.h>
+
 #include <map>
 
+#include <faiss/IndexBinaryFlat.h>
+#include <faiss/IndexBinaryHNSW.h>
+#include <faiss/IndexBinaryIVF.h>
+#include <faiss/IndexHNSW.h>
+#include <faiss/IndexIDMap.h>
+#include <faiss/IndexIVFFlat.h>
+#include <faiss/IndexIVFPQFastScan.h>
+#include <faiss/IndexLSH.h>
+#include <faiss/IndexNSG.h>
+#include <faiss/IndexPQFastScan.h>
+#include <faiss/IndexPreTransform.h>
+#include <faiss/IndexRefine.h>
+
 namespace faiss {
 
 namespace {
@@ -122,6 +136,11 @@ std::string reverse_index_factory(const faiss::Index* index) {
             const faiss::IndexHNSW* hnsw_index =
                     dynamic_cast<const faiss::IndexHNSW*>(index)) {
         return "HNSW" + std::to_string(get_hnsw_M(hnsw_index));
+    } else if (
+            const faiss::IndexNSG* nsg_index =
+                    dynamic_cast<const faiss::IndexNSG*>(index)) {
+        return "NSG" + std::to_string(nsg_index->nsg.R) + "," +
+                reverse_index_factory(nsg_index->storage);
     } else if (
             const faiss::IndexRefine* refine_index =
                     dynamic_cast<const faiss::IndexRefine*>(index)) {
diff --git a/faiss/cppcontrib/factory_tools.h b/faiss/cppcontrib/factory_tools.h
index f83a6db4ad..20b9237254 100644
--- a/faiss/cppcontrib/factory_tools.h
+++ b/faiss/cppcontrib/factory_tools.h
@@ -9,20 +9,13 @@
 
 #pragma once
 
-#include <faiss/IndexBinaryFlat.h>
-#include <faiss/IndexBinaryHNSW.h>
-#include <faiss/IndexBinaryIVF.h>
-#include <faiss/IndexHNSW.h>
-#include <faiss/IndexIDMap.h>
-#include <faiss/IndexIVFFlat.h>
-#include <faiss/IndexIVFPQFastScan.h>
-#include <faiss/IndexLSH.h>
-#include <faiss/IndexPQFastScan.h>
-#include <faiss/IndexPreTransform.h>
-#include <faiss/IndexRefine.h>
+#include <string>
 
 namespace faiss {
 
+struct Index;
+struct IndexBinary;
+
 std::string reverse_index_factory(const faiss::Index* index);
 std::string reverse_index_factory(const faiss::IndexBinary* index);
 
diff --git a/tests/test_factory_tools.cpp b/tests/test_factory_tools.cpp
index 2e77645e80..f5dda2ad68 100644
--- a/tests/test_factory_tools.cpp
+++ b/tests/test_factory_tools.cpp
@@ -24,6 +24,8 @@ TEST(TestFactoryTools, TestReverseIndexFactory) {
                  "HNSW32",
                  "SQ8",
                  "SQfp16",
+                 "NSG24,Flat",
+                 "NSG16,SQ8",
          }) {
         std::unique_ptr<Index> index{index_factory(64, factory)};
         ASSERT_TRUE(index);
@@ -32,6 +34,8 @@ TEST(TestFactoryTools, TestReverseIndexFactory) {
     using Case = std::pair<const char*, const char*>;
     for (auto [src, dst] : {
                  Case{"SQ8,RFlat", "SQ8,Refine(Flat)"},
+                 Case{"NSG", "NSG32,Flat"},
+                 Case{"NSG,PQ8", "NSG32,PQ8x8"},
          }) {
         std::unique_ptr<Index> index{index_factory(64, src)};
         ASSERT_TRUE(index);