diff --git a/js/meshopt_simplifier.js b/js/meshopt_simplifier.js
index 16a710fb..7f5838cc 100644
--- a/js/meshopt_simplifier.js
+++ b/js/meshopt_simplifier.js
@@ -278,7 +278,8 @@ var MeshoptSimplifier = (function () {
 			assert(vertex_attributes instanceof Float32Array);
 			assert(vertex_attributes.length % vertex_attributes_stride == 0);
 			assert(vertex_attributes_stride >= 0);
-			assert(vertex_lock == null || vertex_lock.length == vertex_positions.length);
+			assert(vertex_lock == null || vertex_lock instanceof Uint8Array);
+			assert(vertex_lock == null || vertex_lock.length == vertex_positions.length / vertex_positions_stride);
 			assert(target_index_count >= 0 && target_index_count <= indices.length);
 			assert(target_index_count % 3 == 0);
 			assert(target_error >= 0);
diff --git a/js/meshopt_simplifier.module.d.ts b/js/meshopt_simplifier.module.d.ts
index c8b7763e..06686f73 100644
--- a/js/meshopt_simplifier.module.d.ts
+++ b/js/meshopt_simplifier.module.d.ts
@@ -27,7 +27,7 @@ export const MeshoptSimplifier: {
 		vertex_attributes: Float32Array,
 		vertex_attributes_stride: number,
 		attribute_weights: number[],
-		vertex_lock: boolean[] | null,
+		vertex_lock: Uint8Array | null,
 		target_index_count: number,
 		target_error: number,
 		flags?: Flags[]
diff --git a/js/meshopt_simplifier.module.js b/js/meshopt_simplifier.module.js
index 7f183f92..8c3a79d3 100644
--- a/js/meshopt_simplifier.module.js
+++ b/js/meshopt_simplifier.module.js
@@ -277,7 +277,8 @@ var MeshoptSimplifier = (function () {
 			assert(vertex_attributes instanceof Float32Array);
 			assert(vertex_attributes.length % vertex_attributes_stride == 0);
 			assert(vertex_attributes_stride >= 0);
-			assert(vertex_lock == null || vertex_lock.length == vertex_positions.length);
+			assert(vertex_lock == null || vertex_lock instanceof Uint8Array);
+			assert(vertex_lock == null || vertex_lock.length == vertex_positions.length / vertex_positions_stride);
 			assert(target_index_count >= 0 && target_index_count <= indices.length);
 			assert(target_index_count % 3 == 0);
 			assert(target_error >= 0);
diff --git a/js/meshopt_simplifier.test.js b/js/meshopt_simplifier.test.js
index a2a7e0bc..822b4e60 100644
--- a/js/meshopt_simplifier.test.js
+++ b/js/meshopt_simplifier.test.js
@@ -114,6 +114,23 @@ var tests = {
 		assert.deepEqual(res[0], expected);
 	},
 
+	simplifyLockFlags: function () {
+		// 0
+		// 1 2
+		// 3 4 5
+		var indices = new Uint32Array([0, 2, 1, 1, 2, 3, 3, 2, 4, 2, 5, 4]);
+
+		var positions = new Float32Array([0, 2, 0, 0, 1, 0, 1, 1, 0, 0, 0, 0, 1, 0, 0, 2, 0, 0]);
+		var locks = new Uint8Array([1, 1, 1, 1, 0, 1]); // only vertex 4 can move
+
+		var res = simplifier.simplifyWithAttributes(indices, positions, 3, new Float32Array(), 1, [], locks, 3, 0.01);
+
+		var expected = new Uint32Array([0, 2, 1, 1, 2, 3, 2, 5, 3]);
+
+		assert.deepEqual(res[0], expected);
+		assert.equal(res[1], 0); // error
+	},
+
 	getScale: function () {
 		var positions = new Float32Array([0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 3]);