attempt to get length of local 'str' (a nil value)

[verymilan]

ServerError: Lua: Runtime error from mod 'ambience' in callback on_playerReceiveFields(): ...minetest/minetest/bin/../builtin/common/misc_helpers.lua:166: attempt to get length of local 'str' (a nil value)
stack traceback:
    ...minetest/minetest/bin/../builtin/common/misc_helpers.lua:166: in function 'split'
    /var/minetest/illuna_world/worldmods/ambience/init.lua:643: in function </var/minetest/illuna_world/worldmods/ambience/init.lua:637>
    /home/minetest/minetest/bin/../builtin/game/register.lua:369: in function </home/minetest/minetest/bin/../builtin/game/register.lua:349>

[verymilan]

Is it possible that this happens while a player tries to change the sound value while no sound is playing?