Lua Recieving soul on frag

[Denis Tacador De Runa]

function onDeath(cid, corpse, killer)
doPlayerSendTextMessage(killer, MESSAGE_STATUS_CONSOLE_BLUE, "You took " .. getPlayerName(cid) .. "'s soul.")
doPlayerAddSoul(killer, 1)
end

Does anyone know why this does not add one soul point to the killer?
I tried it with using to add a white pearl instead of soul points but it worked perfectly fine, but doesn't work at all with soul points.