order by nested projection implicit alias not covering

create index ix10 on default(c1, addr.fname);
EXPLAIN SELECT addr.fname
FROM default
WHERE c1 = 10
ORDER BY fname;

Above statement is not covering. Below is covering.

EXPLAIN SELECT addr.fname AS fname
FROM default
WHERE c1 = 10
ORDER BY fname;

EXPLAIN SELECT addr.fname
FROM default
WHERE c1 = 10
ORDER BY addr.fname;