Encounters ElementNotVisibleExceptions when using HtmlUnit as Browser

[bertanasco]

Running madcow 2.0 against a bootstrap + backbone.js application always encounter the exception below :

org.openqa.selenium.ElementNotVisibleException: You may only interact with visible elements

madcow-config.xml

   <execution>
        <runner>
            <type>au.com.ps4impact.madcow.runner.webdriver.WebDriverStepRunner</type>
            <parameters>
                <!--<browser>Firefox</browser>-->
                <browser>HtmlUnit</browser>
            </parameters>
        </runner>
        <env.default>DEV</env.default>
    </execution>

screenshot showing that element is visible

note: This isn't the case when using "Chrome"