Brute force with custom charset

Hi. I am trying to launch a brute-force attack on a WordPress hash and i know that the password contains only letters and numbers and has length of 6-9 signs. From the wiki i figured out that i have to type this command into my oclHashcat:

oclHashcat64.exe -m400 -a 3 hash.txt -1 ?l?d

But when i do so hashcat launches brute-force attack with all Built-in charsets: ?l?u?d?s (and not the ones i need it to brute force which is ?l?d), i figured that out from the time it takes to finish the certain lengths of a password. I don't think that it takes absolutely identical amount of time to check 5 signs password in these modes "?l?d" "?l?u?d?s". Could you please indicate what do i do wrong and what exactly should i type to make a brute-force attack only with letters and digits?

Thanks in advance!