ZADD return error on lowercase arguments

Expected result:

127.0.0.1:6379> zadd key nx 3 val3
(integer) 1

actual:

127.0.0.1:6379> zadd key nx 3 val3
(error) ERR syntax error

while

127.0.0.1:6379> zadd key NX 3 val3
(integer) 1

I suspect this is a more general issue