gtting a value from a related record

I have loaded the fulfillment record
        var rec                = record.load({        type            : record.Type.ITEM_FULFILLMENT,
                                                id            : transactionId,
                                                isDynamic    : true        });


Then I want to get fields from the createdfrom.

Isn't it like this?  Which does not return anything... Yes there is data.

        params.endUserId            = rec.getValue({    fieldId        : 'createdfrom.custbody_end_user' });