How to add a IPv6 suffix mask to a nftables set?

Installing and Using OpenWrt Network and Wireless Configuration
1

My set define like this

        set lan_ac_black_ipv6s {
                type ipv6_addr
                flags interval
                auto-merge
                elements = { 
                             fc00::/7,
                             fe80::/10 }
        }

At home network IPv6 prefix is dynamic, so it's popular to setup firewall with suffix mask

When add a element to this set with this command, it return a syntax error

nft add element inet fw4 lan_ac_black_ipv6s { be00::/-64 }

Error: syntax error, unexpected -, expecting number
add element inet fw4 lan_ac_black_ipv6s { be00::/-64 }
^
Does it possible to add an IPv6 suffix mask IP address to a set?If so, how?