diff --git a/examples/counting-world-program.rkt b/examples/counting-world-program.rkt
index 91cb7aa..91791a4 100644
--- a/examples/counting-world-program.rkt
+++ b/examples/counting-world-program.rkt
@@ -11,7 +11,7 @@ handler
 (big-bang 1
           (on-tick add1 1)
           ;;(on-tick (lambda (w) (* w 2)) 1)
-          ;;(stop-when (lambda (w) (> w 10)))
+          (stop-when (lambda (w) (> w 10)))
           )
 
 
diff --git a/image/private/js-impl.js b/image/private/js-impl.js
index 532c286..5c38e53 100644
--- a/image/private/js-impl.js
+++ b/image/private/js-impl.js
@@ -270,15 +270,17 @@ EXPORTS['image-url'] =
         1,
         function(MACHINE) {
             var url = checkString(MACHINE, 'image-url', 0);
+            var oldArgcount = MACHINE.argcount;
             PAUSE(
                 function(restart) {
                     var rawImage = new Image();
                     rawImage.onload = function() {
                         restart(function(MACHINE) {
-                                        finalizeClosureCall(
-                                            MACHINE, 
-                                            makeFileImage(url.toString(),
-                                                          rawImage));
+                            MACHINE.argcount = oldArgcount;
+                            finalizeClosureCall(
+                                MACHINE, 
+                                makeFileImage(url.toString(),
+                                              rawImage));
                         });
                     };
                     rawImage.onerror = function(e) {
diff --git a/js-assembler/runtime-src/runtime.js b/js-assembler/runtime-src/runtime.js
index 92a1ab0..79229c6 100644
--- a/js-assembler/runtime-src/runtime.js
+++ b/js-assembler/runtime-src/runtime.js
@@ -456,6 +456,8 @@ if(this['plt'] === undefined) { this['plt'] = {}; }
             if(--MACHINE.callsBeforeTrampoline < 0) { 
                 throw arguments.callee; 
             }
+            var oldArgcount = MACHINE.argcount;
+
 	    var elt = MACHINE.env[MACHINE.env.length - 1];
 	    var outputPort = 
 		MACHINE.params.currentOutputPort;
@@ -463,6 +465,7 @@ if(this['plt'] === undefined) { this['plt'] = {}; }
 		outputPort.writeDomNode(MACHINE, toDomNode(elt, 'print'));
 		outputPort.writeDomNode(MACHINE, toDomNode("\n", 'display'));
 	    }
+            MACHINE.argcount = oldArgcount;
             return finalizeClosureCall(MACHINE, VOID);
 	},
 	1,
diff --git a/world/js-impl.js b/world/js-impl.js
index f63bd05..82924bf 100644
--- a/world/js-impl.js
+++ b/world/js-impl.js
@@ -1,7 +1,6 @@
 var PAUSE = plt.runtime.PAUSE;
 var makeClosure = plt.baselib.functions.makeClosure;
 var makeRational = plt.baselib.numbers.makeRational;
-var finalizeClosureCall = plt.baselib.functions.finalizeClosureCall;
 var makePrimitiveProcedure = plt.baselib.functions.makePrimitiveProcedure;
 
 
@@ -42,7 +41,6 @@ EXPORTS['big-bang'] =
             var initialWorldValue = MACHINE.env[MACHINE.env.length - 1];
 	    var handlers = [];
 	    for (var i = 1; i < MACHINE.argcount; i++) {
-		// FIXME: typecheck for configuration options
 		handlers.push(checkHandler(MACHINE, 'big-bang', i));
 	    }
 	    bigBang(MACHINE, initialWorldValue, handlers);
@@ -74,20 +72,18 @@ EXPORTS['to-draw'] =
         function(MACHINE) {
             var f = checkProcedure1(MACHINE, "on-tick", 0);
             return new OnDraw(f);
-            // FILL ME IN
         });
 
 
 
 
-
 EXPORTS['stop-when'] =
     makePrimitiveProcedure(
         'stop-when',
         1,
         function(MACHINE) {
             var f = checkProcedure1(MACHINE, "on-tick", 0);
-            // FILL ME IN
+            return new StopWhen(f);
         });
 
 
diff --git a/world/kernel.js b/world/kernel.js
index 4937ec3..d3c1819 100644
--- a/world/kernel.js
+++ b/world/kernel.js
@@ -21,6 +21,8 @@ var finalizeClosureCall = plt.baselib.functions.finalizeClosureCall;
 
 var bigBang = function(MACHINE, initW, handlers) {
 
+    var oldArgcount = MACHINE.argcount;
+
     var toplevelNode = $('<div/>').css('border', '2').appendTo(document.body);
 
     var configs = [];
@@ -59,6 +61,7 @@ var bigBang = function(MACHINE, initW, handlers) {
 
 
 		restart(function(MACHINE) {
+                    MACHINE.argcount = oldArgcount;
 		    finalizeClosureCall(
 			MACHINE, 
 			finalWorldValue);
@@ -222,9 +225,30 @@ DefaultOnDraw.prototype.toRawHandler = function(MACHINE) {
 
 
 
+var StopWhen = function(handler) {
+    WorldConfigOption.call(this, 'stop-when');
+    this.handler = handler;
+};
 
+StopWhen.prototype = plt.baselib.heir(WorldConfigOption.prototype);
 
-
+StopWhen.prototype.toRawHandler = function(MACHINE) {
+    var that = this;
+    var worldFunction = function(world, k) { 
+        plt.baselib.functions.internalCallDuringPause(
+            MACHINE,
+            that.handler,
+            function(v) {
+                k(v);
+            },
+            
+            function(err) {
+                console.log(err);
+            },
+            world);
+    }
+    return rawJsworld.stop_when(worldFunction);
+};