var fieldLookUp = search.lookupFields({
    type: search.Type.SALES_ORDER,
    id: transactionID,
    columns: ['location', 'shippingAddress.internalid']
});

console.log(fieldLookUp.location[0].value); //This works for location
console.log(fieldLookUp.shippingAddress.internalid[0].value); //this doesn't work. 

Here is what the fieldLookUp contains
 {
"location": [
                {
                    "value": "2",
                    "text": "FR-NB"
                }
            ],
"shippingAddress.internalid": [
                {
                    "value": "218061",
                    "text": "218061"
                }}