How to set static mac address on VAP

Hi, 

I have a fortigate model 100D (version v5.4.4,build6003 (GA)) and 60D (version v5.4.4,build6003 (GA))  and it is a switch controller. I created a SSID and this SSID generated a mac-address Virtual (VAP). 

I realized that all times that I reboot my firewall, the mac-address change.

Anyone know how to set a static mac-address on SSID?