How to refer to a column after join operation

[Androidown]

To say, I have a relation which is created from a join operation.

from a=invoices
join b=customers (a.invoice_id == b.customer_id)

I want to select column field of that relation, but somehow I don't know which relation field belongs to, what I do know is column field does exist in that joined relation.
I was expecting the following prql to compile.

from a=invoices
join b=customers (a.invoice_id == b.customer_id)
select {field}

Expected SQL:

with table_0 as (
SELECT
  a.*,
  b.*
FROM
  invoices AS a
  JOIN customers AS b ON a.invoice_id = b.customer_id
)
SELECT
   field
FROM
  table_0 

But it fails with error:

Error: 
   ╭─[:3:9]
   │
 3 │ select {field}
   │         ──┬──  
   │           ╰──── Ambiguous name
   │ 
   │ Help: could be any of: a.field, b.field
───╯