diff --git a/index.js b/index.js
index 9549c96..6e82175 100644
--- a/index.js
+++ b/index.js
@@ -175,12 +175,21 @@ function hasKey (obj, keys) {
 
 function setKey (obj, keys, value) {
     var o = obj;
-    keys.slice(0,-1).forEach(function (key) {
+    for (var i = 0; i < keys.length-1; i++) {
+        var key = keys[i];
+        if (key === '__proto__') return;
         if (o[key] === undefined) o[key] = {};
+        if (o[key] === Object.prototype || o[key] === Number.prototype
+            || o[key] === String.prototype) o[key] = {};
+        if (o[key] === Array.prototype) o[key] = [];
         o = o[key];
-    });
-    
+    }
+
     var key = keys[keys.length - 1];
+    if (key === '__proto__') return;
+    if (o === Object.prototype || o === Number.prototype
+        || o === String.prototype) o = {};
+    if (o === Array.prototype) o = [];
     if (o[key] === undefined || typeof o[key] === 'boolean') {
         o[key] = value;
     }
diff --git a/package.json b/package.json
index 5935fc3..3cf58de 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
     "name": "minimist",
-    "version": "0.2.0",
+    "version": "0.2.1",
     "description": "parse argument options",
     "main": "index.js",
     "devDependencies": {
diff --git a/test/proto.js b/test/proto.js
new file mode 100644
index 0000000..8649107
--- /dev/null
+++ b/test/proto.js
@@ -0,0 +1,44 @@
+var parse = require('../');
+var test = require('tape');
+
+test('proto pollution', function (t) {
+    var argv = parse(['--__proto__.x','123']);
+    t.equal({}.x, undefined);
+    t.equal(argv.__proto__.x, undefined);
+    t.equal(argv.x, undefined);
+    t.end();
+});
+
+test('proto pollution (array)', function (t) {
+    var argv = parse(['--x','4','--x','5','--x.__proto__.z','789']);
+    t.equal({}.z, undefined);
+    t.deepEqual(argv.x, [4,5]);
+    t.equal(argv.x.z, undefined);
+    t.equal(argv.x.__proto__.z, undefined);
+    t.end();
+});
+
+test('proto pollution (number)', function (t) {
+    var argv = parse(['--x','5','--x.__proto__.z','100']);
+    t.equal({}.z, undefined);
+    t.equal((4).z, undefined);
+    t.equal(argv.x, 5);
+    t.equal(argv.x.z, undefined);
+    t.end();
+});
+
+test('proto pollution (string)', function (t) {
+    var argv = parse(['--x','abc','--x.__proto__.z','def']);
+    t.equal({}.z, undefined);
+    t.equal('...'.z, undefined);
+    t.equal(argv.x, 'abc');
+    t.equal(argv.x.z, undefined);
+    t.end();
+});
+
+test('proto pollution (constructor)', function (t) {
+    var argv = parse(['--constructor.prototype.y','123']);
+    t.equal({}.y, undefined);
+    t.equal(argv.y, undefined);
+    t.end();
+});