Transforming an Invoice to a Creditmemo

I'm running this :

var gue_creditmemo = nlapiTransformRecord('invoice', g_invoice_id, 'creditmemo');

Then I'm wanting to change the value of a particular (noninventory) item.

gue_creditmemo.setLineItemValue('item', 'amount', x, property_val);  nlapiCommitLineItem('item');

Which is fine... until I submit the record :

gue_creditmemo_id = nlapiSubmitRecord(gue_creditmemo, true);

Then I see : "You cannot apply more than your total credit." ???

Can anyone direct me how I can resolve this in code or why I'm seeing it (beyond the pretty obvious English message ) please?
Thanks in advance
Neil