Error - Npc interface

Hello, I've got this error message:
[20/02/2018 13:22:30] [Error - Npc interface]
[20/02/2018 13:22:31] (Unknown script file)
[20/02/2018 13:22:31] Description:
[20/02/2018 13:22:31] data/npc/lib/npcsystem/modules.lua:1229: bad argument #2 to 'error' (number expected, got string)
[20/02/2018 13:22:31] stack traceback:
[20/02/2018 13:22:31] [C]: in function 'error'
[20/02/2018 13:22:31] data/npc/lib/npcsystem/modules.lua:1229: in function 'callbackOnSell'
[20/02/2018 13:22:31] data/npc/lib/npcsystem/npchandler.lua:265: in function 'processModuleCallback'
[20/02/2018 13:22:31] data/npc/lib/npcsystem/npchandler.lua:450: in function 'onSell'
[20/02/2018 13:22:31] data/npc/lib/npcsystem/modules.lua:1296: in function <data/npc/lib/npcsystem/modules.lua:1295>
And there is a part from .lua file: shopModule:addSellableItem({"straw hat"}, 12754, 100, "straw hat")
I can add that I also have this problem for buying items from NPC but it disappeared while I deleted "last" straw hat.
Now I am hopeless maybe you guys has some ideas