diff --git a/index.js b/index.js
index 79c9f79..ce507da 100644
--- a/index.js
+++ b/index.js
@@ -106,10 +106,14 @@ function copy(src) {
 	return src;
 }
 
-function walk(root, cb, immutable) {
+var emptyNull = { __proto__: null };
+
+function walk(root, cb) {
 	var path = [];
 	var parents = [];
 	var alive = true;
+	var options = arguments.length > 2 ? arguments[2] : emptyNull;
+	var immutable = !!options.immutable;
 
 	return (function walker(node_) {
 		var node = immutable ? copy(node_) : node_;
@@ -268,12 +272,14 @@ Traverse.prototype.set = function (ps, value) {
 	return value;
 };
 
+var immutableOpts = { __proto__: null, immutable: true };
+
 Traverse.prototype.map = function (cb) {
-	return walk(this.value, cb, true);
+	return walk(this.value, cb, immutableOpts);
 };
 
 Traverse.prototype.forEach = function (cb) {
-	this.value = walk(this.value, cb, false);
+	this.value = walk(this.value, cb);
 	return this.value;
 };
 
diff --git a/package.json b/package.json
index c476a73..223335e 100644
--- a/package.json
+++ b/package.json
@@ -16,7 +16,7 @@
 		"in-publish": "^2.0.1",
 		"npmignore": "^0.3.1",
 		"safe-publish-latest": "^2.0.0",
-		"tape": "^5.7.2"
+		"tape": "^5.7.5"
 	},
 	"scripts": {
 		"prepack": "npmignore --auto --commentLines=autogenerated",
@@ -93,8 +93,8 @@
 	},
 	"dependencies": {
 		"gopd": "^1.0.1",
-		"typedarray.prototype.slice": "^1.0.1",
-		"which-typed-array": "^1.1.11"
+		"typedarray.prototype.slice": "^1.0.2",
+		"which-typed-array": "^1.1.15"
 	},
 	"engines": {
 		"node": ">= 0.4"