JSON array

I'm trying to get put some data into a div which is retrieved from a JSON request via ajax and jQuery:-

	$(document).ready(function () {
		$.ajax({
			type: 'GET',
			url: 'getTransactions.php?SessionID=53&AccountID=45565',
			dataType: 'json',
			success: function(retHtml, textStatus) {
			    var obj = jQuery.parseJSON(retHtml);
                $.each(obj, function(AccountID, transactionID, Date, transaction, Status, Amount, CountOfMemo) {
                    $('#divTransactions').append(AccountID + '<br/>');
                    $('#divTransactions').append(transactionID + '<br/>');
                    $('#divTransactions').append(Date + '<br/>');
                    $('#divTransactions').append(transaction + '<br/>');
                    $('#divTransactions').append(Status + '<br/>');
                    $('#divTransactions').append(Amount + '<br/>');
                    $('#divTransactions').append(CountOfMemo + '<br/><hr/>');
                    });
				},
			error: function (xhr, textStatus, errorThrown) {
				alert(xhr + "\n" + textStatus + "\n" + errorThrown);
				}
			});
    	});

When I look at retHtml it seems to have pulled the JSON array back, however when the debugger tries to run 'var obj = jQuery.parseJSON(retHtml);' I get

Error: Unable to get property 'length' of undefined or null reference

The page which returns the JSON request (getTransactions.php) gives:-

[
{"AccountID":"66dcf4ef-4c8b-11e2-aa23-180373676ac7","transactionID":"39528e34-4c8b-11e2-aa23-180373676ac7","Date":"2012-12-24 00:00:00","transaction":"BILL PAYMENT","Status":"Cleared","Amount":"-350.07","CountOfMemo":"0"},
{"AccountID":"66dcf4ef-4c8b-11e2-aa23-180373676ac7","transactionID":"39528e3a-4c8b-11e2-aa23-180373676ac7","Date":"2012-12-22 00:00:00","transaction":"CARD PAYMENT","Status":"Cleared","Amount":"-6.33","CountOfMemo":"0"},
{"AccountID":"66dcf4ef-4c8b-11e2-aa23-180373676ac7","transactionID":"39528e35-4c8b-11e2-aa23-180373676ac7","Date":"2012-12-22 00:00:00","transaction":"CARD PAYMENT","Status":"Cleared","Amount":"-50.97","CountOfMemo":"0"}
]

Everything looks fine, I just cant get retHtml to get into an array correctly.

Any ideas what Im doing wrong?