UNIT MQTT: unable to set MAC address

[shiomee]

Hi,
This is a question about UNIT MQTT (SKU:U129).
I sent the following AT command to change the Unit's MAC address, but an error was returned.
What did i do wrong?

b'AT+NETMAC=00-02-CB-12-34-56\r\n'
b'+NETMAC=ERROR:Error Param\r\n'

Other commands can be executed, but only the MAC address cannot be set.

<firmware version>
+VERSION=OK:1.6

Thanks