SDK.REST and how to retrieve EntityReference Id,Name and Logical Name

I am not sure what I am doing wrong but I am trying to get the CustomerId Id, Name and LogicalName from the salesorder entity. I am unable to get the values. Please help. I can get the none EntityReference values.

function retrieveOrder(orderId) {
    SDK.REST.retrieveRecord(
        orderId,
        "SalesOrder",
        null, null,
        function (order) {
             alert(order.CustomerId[0].Id);

             alert(order.CustomerId[0].Name);

           alert(order.CustomerId[0].LogicalName);

        },
        errorHandler
      );
}