nlapiloadrecord for custom record

I cant seem to load a custom record from a user script.  I have a custom column that I look up, which returns an internal id of the custom record.  I need to get at the text value of that rather than the internal id.  How do I do that? In the documentation it says you can load custom records but doesnt explain how and each time I try I get an error: INVALID_RECORD_TYPE

code snippet #1:
var productID_internal=itemRecord.getFieldValue('custitem_product_line');
var productIDRecord=nlapiLoadRecord('custitem_product_line', productID_internal);

Also tried:
var productIDRecord=nlapiLoadRecord('customrecord', productID_internal);