ipset hash:mac iptables

Hi. I'm trying to use ipset to control groups of clients of my ASUS RT-AC68U, with FW 380.64_2, but, when I enter "ipset create MySet hash:mac" on the telnet console, it hangs: it doesn't return the prompt and the process remains in the "ps" listing without being killed by "kill -9 <procNum>"...