How to disable DHCP IPv6 server ?

[Pierre Couderc]

I have the message :

Jan 15 17:25:02 server isc-dhcp-server[1307]: Launching both IPv4 and 
IPv6 servers (please configure INTERFACES in 
/etc/default/isc-dhcp-server if you...

but I have found no doc to explain how to "configure INTERFACES in 
/etc/default/isc-dhcp-server".

I have tried to comment ##INTERFACESv6="" line, but without success.

I am under debian stretch.

Thanks

PC.