diff --git a/index.js b/index.js
index 3446ee2..3aa6790 100644
--- a/index.js
+++ b/index.js
@@ -16,13 +16,42 @@ var forEach = function (xs, fn) {
     }
 };
 
+var defineProp = (function() {
+    try {
+        Object.defineProperty({}, '_', {});
+        return function(obj, name, value) {
+            Object.defineProperty(obj, name, {
+                writable: true,
+                enumerable: false,
+                configurable: true,
+                value: value
+            })
+        };
+    } catch(e) {
+        return function(obj, name, value) {
+            obj[name] = value;
+        };
+    }
+}());
+
+var globals = ['Array', 'Boolean', 'Date', 'Error', 'EvalError', 'Function',
+'Infinity', 'JSON', 'Math', 'NaN', 'Number', 'Object', 'RangeError',
+'ReferenceError', 'RegExp', 'String', 'SyntaxError', 'TypeError', 'URIError',
+'decodeURI', 'decodeURIComponent', 'encodeURI', 'encodeURIComponent', 'escape',
+'eval', 'isFinite', 'isNaN', 'parseFloat', 'parseInt', 'undefined', 'unescape'];
+
+function Context() {}
+Context.prototype = {};
+
 var Script = exports.Script = function NodeScript (code) {
     if (!(this instanceof Script)) return new Script(code);
     this.code = code;
 };
 
-Script.prototype.runInNewContext = function (context) {
-    if (!context) context = {};
+Script.prototype.runInContext = function (context) {
+    if (!(context instanceof Context)) {
+        throw new TypeError("needs a 'context' argument.");
+    }
     
     var iframe = document.createElement('iframe');
     if (!iframe.style) iframe.style = {};
@@ -35,7 +64,12 @@ Script.prototype.runInNewContext = function (context) {
     forEach(Object_keys(context), function (key) {
         win[key] = context[key];
     });
-     
+    forEach(globals, function (key) {
+        if (context[key]) {
+            win[key] = context[key];
+        }
+    });
+
     if (!win.eval && win.execScript) {
         // win.eval() magically appears when this is called in IE:
         win.execScript('null');
@@ -53,6 +87,12 @@ Script.prototype.runInNewContext = function (context) {
             context[key] = win[key];
         }
     });
+
+    forEach(globals, function (key) {
+        if (!(key in context)) {
+            defineProp(context, key, win[key]);
+        }
+    });
     
     document.body.removeChild(iframe);
     
@@ -63,11 +103,15 @@ Script.prototype.runInThisContext = function () {
     return eval(this.code); // maybe...
 };
 
-Script.prototype.runInContext = function (context) {
-    // seems to be just runInNewContext on magical context objects which are
-    // otherwise indistinguishable from objects except plain old objects
-    // for the parameter segfaults node
-    return this.runInNewContext(context);
+Script.prototype.runInNewContext = function (context) {
+    var ctx = Script.createContext(context);
+    var res = this.runInContext(ctx);
+
+    forEach(Object_keys(ctx), function (key) {
+        context[key] = ctx[key];
+    });
+
+    return res;
 };
 
 forEach(Object_keys(Script.prototype), function (name) {
@@ -82,9 +126,7 @@ exports.createScript = function (code) {
 };
 
 exports.createContext = Script.createContext = function (context) {
-    // not really sure what this one does
-    // seems to just make a shallow copy
-    var copy = {};
+    var copy = new Context();
     if(typeof context === 'object') {
         forEach(Object_keys(context), function (key) {
             copy[key] = context[key];