ubable to click on input button in htmlunit using JAVA, when I try following code it gives me exception "TypeError: Cannot read property "mousedown" from null"

```
    HtmlSubmitInput search_button = (HtmlSubmitInput) page.getByXPath(".//*[@id='ctl00_ContentPlaceHolder1_PortalPageControl1_ctl08_PSCDocumentSearchControl1_searchButton']").get(0);
        HtmlPage results_page = search_button.click();

```

I have also tries to cast button into **HtmlButton** and **HtmlElement**

can any one tell me that what would be best cast for input type "submit"