diff --git a/demos/async.html b/demos/async.html
index d166a080..cf1e28a0 100644
--- a/demos/async.html
+++ b/demos/async.html
@@ -19,8 +19,16 @@
         var req = new XMLHttpRequest();
         req.open('GET', href, true);
         req.onreadystatechange = function() {
-          if (req.readyState == 4 && req.status == 200) {
-            callback(req.responseText);
+          if (req.readyState == 4) {
+            if(req.status == 200) {
+              callback(req.responseText);
+            } else {
+              var error = interpreter.createObject(interpreter.ERROR);
+              interpreter.setProperty(error, 'message',
+                  req.status + req.statusText,
+                  Interpreter.NONENUMERABLE_DESCRIPTOR);
+              callback(undefined, error);
+            }
           }
         };
         req.send(null);
@@ -102,11 +110,20 @@ <h1>JS-Interpreter Async Demo</h1>
   will both do nothing, returning <code>true</code> to indicate that the program
   still has code to execute.</p>
 
+  <p>The callback takes one or two arguments.  If called with one, the
+  supplied value is used as the return value of the asynchronous
+  call.  If called with two, the first is ignored and the second is
+  used as the value of an exception thrown by the call.</p>
+
   <p>Click <em>Parse</em>, then either click <em>Step</em> repeatedly,
   or click <em>Run</em> once.  Open your browser's console for errors.</p>
 
   <p><textarea id="code">
-alert(getXhr('async.txt'));
+try {
+  alert(getXhr('async.txt'));
+} catch(e) {
+  alert('Failed: ' + e);
+}
 </textarea><br>
   <button onclick="parseButton()">Parse</button>
   <button onclick="stepButton()" id="stepButton" disabled="disabled">Step</button>
diff --git a/interpreter.js b/interpreter.js
index 917f10e1..3115f4da 100644
--- a/interpreter.js
+++ b/interpreter.js
@@ -3226,8 +3226,13 @@ Interpreter.prototype['stepCallExpression'] = function(stack, state, node) {
       state.value = func.nativeFunc.apply(state.funcThis_, state.arguments_);
     } else if (func.asyncFunc) {
       var thisInterpreter = this;
-      var callback = function(value) {
-        state.value = value;
+      var callback = function(value, exception) {
+        if (arguments.length <= 1) {
+          state.value = value;
+        } else {
+          thisInterpreter.unwind(Interpreter.Completion.THROW,
+                                 exception, undefined);
+        }
         thisInterpreter.paused_ = false;
       };
       // Force the argument lengths to match, then append the callback.