ADQL parser fails to handle reference to join key in natural join

[pdowler]

SELECT from_column, target_column, key_id
FROM TAP_SCHEMA.key_columns
NATURAL JOIN TAP_SCHEMA.keys
WHERE from_table = 'caom2.ObsCoreEnumField'

With the NATURAL JOIN it should be OK to reference key_id (the natural join key); the current code results is an error: IllegalArgumentException: Column [key_id] is ambiguous.