Javascript problem

I am getting the following exception whenever I try to execute a 
javascript function from my test case.

 java.lang.IllegalStateException: may not run executeScript() from class 
com.meterware.httpunit.javascript.JavaScript$Form


What is the problem? I have added Rhino (js.jar) to my class path and 
enabled scripting too.

Thanks,
Sachin

html:

<input type="hidden" name="state" value="1">
<input onClick='onCancel()' class='cuesButton' id='1tbl' name='Cancel' alt='Cancel' value='Cancel' type='button'>
<input onClick='onNext()' class='cuesButton' id='2tbl' name='Next' alt='Next' value='Next' type='button'>

function onCancel() {

	document.forms[0].action = "installCancel.do";
	document.forms[0].submit();
}

function onNext() {
	var f = document.forms[0];
	f.action = "installOptions.do";
	f.submit();
}


Testcase:
 public void testScenario() throws Exception {
        HttpUnitOptions.clearScriptErrorMessages();
        wc = new WebConversation();
        WebForm forms[] = _wc.getCurrentPage().getForms();
        forms[0].getScriptableObject().runScript("JavaScript","onNext()");
}

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