diff --git a/Assets/VRM/UniVRM/Editor/Tests/NormalizeTests.cs b/Assets/VRM/UniVRM/Editor/Tests/NormalizeTests.cs
new file mode 100644
index 000000000..51510cfb7
--- /dev/null
+++ b/Assets/VRM/UniVRM/Editor/Tests/NormalizeTests.cs
@@ -0,0 +1,89 @@
+using System.Collections.Generic;
+using System.Linq;
+using NUnit.Framework;
+using UnityEngine;
+
+namespace VRM
+{
+    public class NormalizeTests
+    {
+        class BoneMap
+        {
+            public List<Transform> SrcBones = new List<Transform>();
+            public List<Transform> DstBones = new List<Transform>();
+            public Dictionary<Transform, Transform> Map = new Dictionary<Transform, Transform>();
+
+            public void Add(GameObject src, GameObject dst)
+            {
+                SrcBones.Add(src?.transform);
+                if (dst != null)
+                {
+                    DstBones.Add(dst.transform);
+                }
+                if (src != null)
+                {
+                    Map.Add(src?.transform, dst?.transform);
+                }
+            }
+
+            public IEnumerable<BoneWeight> CreateBoneWeight(int vertexCount)
+            {
+                int j = 0;
+                for (int i = 0; i < vertexCount; ++i)
+                {
+                    yield return new BoneWeight
+                    {
+                        boneIndex0 = j++,
+                        boneIndex1 = j++,
+                        boneIndex2 = j++,
+                        boneIndex3 = j++,
+                        weight0 = 0.25f,
+                        weight1 = 0.25f,
+                        weight2 = 0.25f,
+                        weight3 = 0.25f,
+                    };
+                }
+            }
+        }
+
+        [Test]
+        public void MapBoneWeightTest()
+        {
+            {
+                var map = new BoneMap();
+                map.Add(new GameObject("a"), new GameObject("A"));
+                map.Add(new GameObject("b"), new GameObject("B"));
+                map.Add(new GameObject("c"), new GameObject("C"));
+                map.Add(new GameObject("d"), new GameObject("D"));
+                map.Add(null, new GameObject("null"));
+                // map.Add(new GameObject("c"), null); // ありえないので Exception にしてある
+                var boneWeights = map.CreateBoneWeight(64).ToArray();
+                var newBoneWeight = BoneNormalizer.MapBoneWeight(boneWeights, map.Map,
+                    map.SrcBones.ToArray(), map.DstBones.ToArray());
+
+                // 正常系
+                // exception が出なければよい
+            }
+
+            {
+                var map = new BoneMap();
+                map.Add(new GameObject("a"), new GameObject("A"));
+                map.Add(new GameObject("b"), new GameObject("B"));
+                map.Add(new GameObject("c"), new GameObject("C"));
+                map.Add(new GameObject("d"), new GameObject("D"));
+                map.Add(null, new GameObject("null"));
+                // map.Add(new GameObject("c"), null); // ありえないので Exception にしてある
+                var boneWeights = map.CreateBoneWeight(64).ToArray();
+                var newBoneWeight = BoneNormalizer.MapBoneWeight(boneWeights, map.Map,
+                    map.SrcBones.ToArray(), map.DstBones.ToArray());
+
+                // 4 つめが 0 になる
+                Assert.AreEqual(0, newBoneWeight[1].boneIndex0);
+                Assert.AreEqual(0, newBoneWeight[1].weight0);
+                // 5 つめ以降が 0 になる。out of range
+                Assert.AreEqual(0, newBoneWeight[1].boneIndex1);
+                Assert.AreEqual(0, newBoneWeight[1].weight1);
+            }
+        }
+    }
+}
diff --git a/Assets/VRM/UniVRM/Editor/Tests/NormalizeTests.cs.meta b/Assets/VRM/UniVRM/Editor/Tests/NormalizeTests.cs.meta
new file mode 100644
index 000000000..b3822bbd4
--- /dev/null
+++ b/Assets/VRM/UniVRM/Editor/Tests/NormalizeTests.cs.meta
@@ -0,0 +1,11 @@
+fileFormatVersion: 2
+guid: 4014814a6ed5bf84faa36c7b99d6d4b8
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 
diff --git a/Assets/VRM/UniVRM/Scripts/SkinnedMeshUtility/BoneNormalizer.cs b/Assets/VRM/UniVRM/Scripts/SkinnedMeshUtility/BoneNormalizer.cs
index 609549912..b4b51ee25 100644
--- a/Assets/VRM/UniVRM/Scripts/SkinnedMeshUtility/BoneNormalizer.cs
+++ b/Assets/VRM/UniVRM/Scripts/SkinnedMeshUtility/BoneNormalizer.cs
@@ -181,6 +181,13 @@ namespace VRM
         /// <param name="setter"></param>
         static bool CopyOrDropWeight(int[] indexMap, int srcIndex, float weight, Action<int, float> setter)
         {
+            if (srcIndex < 0 || srcIndex >= indexMap.Length)
+            {
+                // ありえるかどうかわからないが BoneWeight.boneIndexN に変な値が入っている. 
+                setter(0, 0);
+                return false;
+            }
+
             var dstIndex = indexMap[srcIndex];
             if (dstIndex != -1)
             {
@@ -204,7 +211,7 @@ namespace VRM
         /// <param name="srcBones">変更前のボーン配列</param>
         /// <param name="dstBones">変更後のボーン配列。除去されたボーンがある場合、変更前より短い</param>
         /// <returns></returns>
-        static BoneWeight[] MapBoneWeight(BoneWeight[] src,
+        public static BoneWeight[] MapBoneWeight(BoneWeight[] src,
             Dictionary<Transform, Transform> boneMap,
             Transform[] srcBones,
             Transform[] dstBones