Kylin's hll counter count "NULL" as a value

Reported by Xie Linjiong, he found Kylin's HyperLogLog count NULL as 1, but Hive count it as 0; Below is a sample:

A. source table:
hive> select * from temp.temp_kylin_null_test_20160429;
OK
10003 NULL
10002 NULL
10001 male

B. hive's query result:
hive> select count(distinct gender)],count from temp.temp_kylin_null_test_20160429 where user_id in (10002,10003);

OK
0 2

C. Kylin's query result:

1 2