generate_series in postgresql no result data in array php

Question

I was trying to retrieve a data result provided by the SQL below

SELECT to_char(generate_series('2016-10-04', '2016-12-31', interval '1 mon'), 'YYYY-MM') AS month

If we can run the said query above, it will return a result of:

month(text)
"2016-10"
"2016-11"
"2016-12"

but in my php code, it does not return any result or values are null

$qr = "SELECT to_char(generate_series('2016-10-04', '2016-12-31', interval '1 mon'), 'YYYY-MM') AS month";
$rs = $this->conn->fetchAll($qr);

print_r($rs);

result:

Array
(
)

other sql queries in my php code works but this one is not, am i missing something here? i need a shade on this T_T