diff --git a/CHANGELOG.md b/CHANGELOG.md
index 269c771..8cf757e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## v3.9.1
+### Bug Fixes
+* Fix crash when the avatar has a skinned mesh with out of bounds bone indices. [(more)](https://github.com/d4rkc0d3r/d4rkAvatarOptimizer/issues/129)
+
 ## v3.9.0
 ### Features
 * Generated shaders and materials are now stripped of all properties that got baked into the shader.
diff --git a/Editor/d4rkAvatarOptimizer.cs b/Editor/d4rkAvatarOptimizer.cs
index 46cb37d..d0b0917 100644
--- a/Editor/d4rkAvatarOptimizer.cs
+++ b/Editor/d4rkAvatarOptimizer.cs
@@ -2242,15 +2242,23 @@ void AddDependency(Transform t, Object obj)
             var meshBones = skinnedMesh.bones;
             var usedBoneIDs = new bool[meshBones.Length];
             var boneWeights = skinnedMesh.sharedMesh.boneWeights;
+            int outOfRangeBoneCount = 0;
+            void MarkUsedBone(int boneIndex) {
+                if (boneIndex < 0 || boneIndex >= meshBones.Length) {
+                    outOfRangeBoneCount++;
+                    return;
+                }
+                usedBoneIDs[boneIndex] = true;
+            }
             for (int i = 0; i < boneWeights.Length; i++)
             {
-                usedBoneIDs[boneWeights[i].boneIndex0] = true;
+                MarkUsedBone(boneWeights[i].boneIndex0);
                 if (boneWeights[i].weight1 > 0) {
-                    usedBoneIDs[boneWeights[i].boneIndex1] = true;
+                    MarkUsedBone(boneWeights[i].boneIndex1);
                     if (boneWeights[i].weight2 > 0) {
-                        usedBoneIDs[boneWeights[i].boneIndex2] = true;
+                        MarkUsedBone(boneWeights[i].boneIndex2);
                         if (boneWeights[i].weight3 > 0) {
-                            usedBoneIDs[boneWeights[i].boneIndex3] = true;
+                            MarkUsedBone(boneWeights[i].boneIndex3);
                         }
                     }
                 }
@@ -2262,6 +2270,10 @@ void AddDependency(Transform t, Object obj)
                     AddDependency(meshBones[i], skinnedMesh);
                 }
             }
+            if (outOfRangeBoneCount > 0)
+            {
+                Debug.LogWarning($"Skinned mesh renderer {GetPathToRoot(skinnedMesh)} has {outOfRangeBoneCount} out of range bone indices");
+            }
         }
         foreach (var behavior in GetComponentsInChildren<Behaviour>(true)
             .Where(b => b != null && (b.GetType().Name.Contains("Constraint") || b.GetType().FullName.StartsWithSimple("RootMotion.FinalIK"))))
diff --git a/package.json b/package.json
index 55fcf1d..2763294 100644
--- a/package.json
+++ b/package.json
@@ -1,7 +1,7 @@
 {
   "name": "d4rkpl4y3r.d4rkavataroptimizer",
   "displayName": "d4rkAvatarOptimizer",
-  "version": "3.9.0",
+  "version": "3.9.1",
   "unity": "2019.4",
   "description": "An optimizer aiming to reduce mesh & material count and more of VRChat 3.0 avatars.",
   "dependencies": {},