diff --git a/CHANGELOG-PRERELEASE.md b/CHANGELOG-PRERELEASE.md
index 23f72e8c8..0d874153f 100644
--- a/CHANGELOG-PRERELEASE.md
+++ b/CHANGELOG-PRERELEASE.md
@@ -16,6 +16,7 @@ The format is based on [Keep a Changelog].
 ### Removed
 
 ### Fixed
+- Error with nested merge skinned mesh `#1340`
 
 ### Security
 
diff --git a/Editor/Processors/SkinnedMeshes/MergeSkinnedMeshProcessor.cs b/Editor/Processors/SkinnedMeshes/MergeSkinnedMeshProcessor.cs
index 8c4e28a52..9e889480d 100644
--- a/Editor/Processors/SkinnedMeshes/MergeSkinnedMeshProcessor.cs
+++ b/Editor/Processors/SkinnedMeshes/MergeSkinnedMeshProcessor.cs
@@ -367,8 +367,8 @@ TexCoordStatus TexCoordStatusMax(TexCoordStatus x, TexCoordStatus y) =>
 
                         foreach (var buffer in buffers)
                         {
-                            buffer.Shapes[newName] = buffer.Shapes[name];
-                            buffer.Shapes.Remove(name);
+                            if (buffer.Shapes.Remove(name, out var shape))
+                                buffer.Shapes[newName] = shape;
                         }
 
                         mappings.Add(($"blendShape.{name}", $"blendShape.{newName}"));
diff --git a/Runtime/assembly-info.cs b/Runtime/assembly-info.cs
index d9f366f32..015dce03a 100644
--- a/Runtime/assembly-info.cs
+++ b/Runtime/assembly-info.cs
@@ -3,3 +3,4 @@
 [assembly:InternalsVisibleTo("com.anatawa12.avatar-optimizer.editor")]
 [assembly:InternalsVisibleTo("com.anatawa12.avatar-optimizer.internal.trace-and-optimize-base")]
 [assembly:InternalsVisibleTo("com.anatawa12.avatar-optimizer.test.basic")]
+[assembly:InternalsVisibleTo("com.anatawa12.avatar-optimizer.test.utils")]
diff --git a/Test~/Basic/MergeSkinnedMeshTest.cs b/Test~/Basic/MergeSkinnedMeshTest.cs
index a3bfc182c..8f77b8dde 100644
--- a/Test~/Basic/MergeSkinnedMeshTest.cs
+++ b/Test~/Basic/MergeSkinnedMeshTest.cs
@@ -1,3 +1,4 @@
+using System.Collections.Generic;
 using Anatawa12.AvatarOptimizer.AnimatorParsersV2;
 using Anatawa12.AvatarOptimizer.ndmf;
 using Anatawa12.AvatarOptimizer.Processors.SkinnedMeshes;
@@ -243,4 +244,55 @@ public void CopySourceAnimationErrorActivenessAnimationOfSourceMismatch()
             new MergeSkinnedMeshProcessor(merged).Process(context, context.GetMeshInfoFor(mergedRenderer));
         });
     }
+
+    [Test]
+    public void NestedMergeSkinnedMeshWithPartialBlendShapes()
+    {
+        var mesh1 = TestUtils.NewCubeMesh();
+        mesh1.AddBlendShapeFrame("mesh1", 100, TestUtils.NewCubeBlendShapeFrame((0, Vector3.up)), null, null);
+        var mesh2 = TestUtils.NewCubeMesh();
+        mesh2.AddBlendShapeFrame("mesh2", 100, TestUtils.NewCubeBlendShapeFrame((0, Vector3.right)), null, null);
+        var mesh3 = TestUtils.NewCubeMesh();
+        mesh3.AddBlendShapeFrame("mesh3", 100, TestUtils.NewCubeBlendShapeFrame((0, Vector3.forward)), null, null);
+
+        var avatar = TestUtils.NewAvatar();
+        var renderer1GameObject = Utils.NewGameObject("Renderer1", avatar.transform);
+        var renderer1 = renderer1GameObject.AddComponent<SkinnedMeshRenderer>();
+        renderer1.sharedMesh = mesh1;
+        var renderer2GameObject = Utils.NewGameObject("Renderer2", avatar.transform);
+        var renderer2 = renderer2GameObject.AddComponent<SkinnedMeshRenderer>();
+        renderer2.sharedMesh = mesh2;
+        var renderer3GameObject = Utils.NewGameObject("Renderer3", avatar.transform);
+        var renderer3 = renderer3GameObject.AddComponent<SkinnedMeshRenderer>();
+        renderer3.sharedMesh = mesh3;
+        var mergedIntermediateGameObject = Utils.NewGameObject("MergedIntermediate", avatar.transform);
+        var mergedIntermediateRenderer = mergedIntermediateGameObject.AddComponent<SkinnedMeshRenderer>();
+        var mergedFinalGameObject = Utils.NewGameObject("MergedFinal", avatar.transform);
+        var mergedFinalRenderer = mergedFinalGameObject.AddComponent<SkinnedMeshRenderer>();
+
+        var context = new BuildContext(avatar, null);
+        context.ActivateExtensionContext<Processors.MeshInfo2Context>();
+        context.ActivateExtensionContext<ObjectMappingContext>();
+        context.ActivateExtensionContext<DestroyTracker.ExtensionContext>();
+
+        var meshInfo1 = context.GetMeshInfoFor(renderer1);
+        var meshInfo2 = context.GetMeshInfoFor(renderer2);
+        var meshInfo3 = context.GetMeshInfoFor(renderer3);
+        var meshInfoIntermediate = context.GetMeshInfoFor(mergedIntermediateRenderer);
+        var meshInfoFinal = context.GetMeshInfoFor(mergedFinalRenderer);
+
+        MergeSkinnedMeshProcessor.DoMerge(context,
+            meshInfoIntermediate,
+            new[] { meshInfo1, meshInfo2 },
+            new[] { new[] { 0 }, new[] { 0 } },
+            new List<(MeshTopology, Material)>() { (MeshTopology.Triangles, null) }
+        );
+
+        MergeSkinnedMeshProcessor.DoMerge(context,
+            meshInfoFinal,
+            new[] { meshInfoIntermediate, meshInfo3 },
+            new[] { new[] { 0 }, new[] { 0 } },
+            new List<(MeshTopology, Material)>() { (MeshTopology.Triangles, null) }
+        );
+    }
 }
diff --git a/Test~/Utils/TestUtils.cs b/Test~/Utils/TestUtils.cs
index f0a9aea1b..102a080b4 100644
--- a/Test~/Utils/TestUtils.cs
+++ b/Test~/Utils/TestUtils.cs
@@ -11,6 +11,8 @@ namespace Anatawa12.AvatarOptimizer.Test
 {
     public static class TestUtils
     {
+        class DummyAvatarTagComponent : AvatarTagComponent {}
+
         public static GameObject NewAvatar(string name = null)
         {
             var root = new GameObject();
@@ -20,6 +22,8 @@ public static GameObject NewAvatar(string name = null)
 #if AAO_VRCSDK3_AVATARS
             var descriptor = root.AddComponent<VRC.SDK3.Avatars.Components.VRCAvatarDescriptor>();
 #endif
+            // for any AvatarTagComponent checks on the avatar
+            root.AddComponent<DummyAvatarTagComponent>();
             return root;
         }
 
@@ -126,6 +130,13 @@ public static Mesh NewCubeMesh()
             mesh.SetSubMesh(0, new SubMeshDescriptor(0, 12 * 3));
             return mesh;
         }
+        
+        public static Vector3[] NewCubeBlendShapeFrame(params (int index, Vector3 delta)[] deltas)
+        {
+            var frame = new Vector3[8];
+            foreach (var (index, delta) in deltas) frame[index] = delta;
+            return frame;
+        }
 
         public static SkinnedMeshRenderer NewSkinnedMeshRenderer(Mesh mesh)
         {