Get html5 data tag

Im trying to get data out of a html5 div:-

<div data-transactionID='22' class='clsTransactionRow'></div>
<div data-transactionID='23' class='clsTransactionRow'></div>
<div data-transactionID='24' class='clsTransactionRow'></div>
<div data-transactionID='25' class='clsTransactionRow'></div>

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I need to be able to get the 'transactionID', so using jQuery I thought I could just add:-

$(".clsTransactionRow").click(function () { alert(jQuery.data($(this), 'data-transactionID')); });

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However, all I get is 'undefined' as my messagebox.

What am I doing wrong?

Thank you