How to resolve unexpected error in fieldChanged when using getCurrentSublistField?

What I'm trying to accomplish: Have the "units" line item field disabled right away upon adding a new line item.

Steps to Repro:

1. Enter Item Name
2. Select the item record

Error when adding a new line item:

Code:

function fieldChanged (context){ 
        var curr = context.currentRecord;


        var sublistIndex = curr.getCurrentSublistIndex({
            sublistId: 'item'
        });
        log.debug({ title: 'sublist index', details: JSON.stringify(sublistIndex) });



        var sublistName = context.sublistId;
        var sublistFieldId = context.fieldId;
        if(sublistName == 'item' && sublistFieldId == 'item'){
            var sublistFieldMetadata = curr.getCurrentSublistField({
                sublistId: 'item',
                fieldId: 'units',
           });
           sublistFieldMetadata.isDisabled = true;
        }
    }


    return {
        //lineInit: disableLineField,
        fieldChanged: fieldChanged
    };