`bad argument #1 to 'sub' (number expected, got nil)` error - fail to join channel

[sofar]

/var/minetest/.minetest/mods/irc/irc/messages.lua:107: bad argument #1 to 'sub' (number expected, got nil)
 stack traceback:
 	[C]: in function 'sub'
    /var/minetest/.minetest/mods/irc/irc/messages.lua:107: in function 'fromRFC1459'
    /var/minetest/.minetest/mods/irc/irc/messages.lua:12: in function 'Message'
    /var/minetest/.minetest/mods/irc/irc/init.lua:174: in function 'think'
    /var/minetest/.minetest/mods/irc/init.lua:131: in function </var/minetest/.minetest/mods/irc/init.lua:131>
[C]: in function 'xpcall'
    /var/minetest/.minetest/mods/irc/init.lua:131: in function </var/minetest/.minetest/mods/irc/init.lua:120>
    /usr/share/minetest/builtin/game/register.lua:429: in function </usr/share/minetest/builtin/game/register.lua:413>

[kaeza]

How do I reproduce it?