Problem adding DHCP option, says option is in use

I'm trying to add DHCP options 44 on Internal hardware switch interface. But when I add DHCP option 44 it gives me an error that says: Option is in use as a DHCP setting: wins-server1, wins-server1

See attachement

DHCP is working fine otherwise and other options seem to work too. There is no other DHCP server configured on the firewall.

Model: FortiGate 60E

Version: v5.6.3 build1547 (GA)