Compound class names are not supported error in WebDriver

I have a method to count the number of elements in divs and to return their number.

 public int getNumberOfOpenBets() {

     openBetsSlip = driver.findElement(By.id("form_open_bets"));
     openBets = openBetsSlip.findElements(By.className(" cashout_noCash"));
     return openBets.size();
 }

That's the page source

<form id="form_open_bets" method="post" name="form_open_bets">
    <input type="hidden" value="" name="action">
    <input type="hidden" value="" name="bet_id">
    <input type="hidden" value="" name="cashout_price">
    <input id="target_page" type="hidden" value="" name="target_page">
    <div id="By.id" class="slipWrapper ">
        <div id="openBets_header"></div>
        <div id="cashout_1626" class=" cashout_noCash">
            <div id="cashout_1625" class=" cashout_noCash">
                <div id="cashout_1615" class=" cashout_noCash">
                    <div id="cashout_1614" class=" cashout_noCash">
                        <div id="cashout_1613" class=" cashout_noCash">
                        </div>
                    </div>
                </div>
            </div>
        </div>
    </div>
</form>

WebDriver is throwing the following error: Compound class names are not supported. Consider searching for one class name and filtering the results or use CSS selectors.

org.openqa.selenium.InvalidSelectorException: Compound class names are not supported. Consider searching for one class name and filtering the results or use CSS selectors.
For documentation on this error, please visit: http://seleniumhq.org/exceptions/invalid_selector_exception.html
Build info: version: '2.31.0', revision: '1bd294d185a80fa4206dfeab80ba773c04ac33c0', time: '2013-02-27 13:51:26'
System info: os.name: 'Windows 7'