Credit Memo / Apply to particular invoice

I am trying to use client SS to check off the related invoice to apply it to.

I can cycle through the items in the Apply tab with SS:

     for (var i = 1; i <= nlapiGetLineItemCount('apply'); i++) {        if (nlapiGetLineItemValue('apply', 'applrefnum', i) == tranID) {          nlapiSetLineItemValue('apply', 'applapply', i, 'T');         }      } 

but the attempt to set the checkbox for that line item fails, but does not seem to return an error.  I know I am finding the right line item as I can add an alert to that conditional check to tell me the iteration value and the details of that line and it's correct.