modules.lua:1229: attempt to compare boolean with number

[waxo]

When trying to buy from any NPC and click too fast on buy an item many times, the user gets a message saying "You can't buy it so fast" and the console shows this error:

[Error - NpcScript Interface]
(Unknown script file)
Description:
data/npc/lib/npcsystem/modules.lua:1229: attempt to compare boolean with number
stack traceback:
    data/npc/lib/npcsystem/modules.lua:1229: in function 'callbackOnBuy'
    data/npc/lib/npcsystem/npchandler.lua:298: in function 'processModuleCallback'
    data/npc/lib/npcsystem/npchandler.lua:504: in function 'onBuy'
    data/npc/lib/npcsystem/modules.lua:1346: in function <data/npc/lib/npcsystem/modules.lua:1345>

Function 'callbackOnBuy' in modules.lua goes as follows (being the line 1229 the one comparing if(a < amount), line 44 here):

function ShopModule:callbackOnBuy(cid, itemid, subType, amount, ignoreCap, inBackpacks)
        local shopItem = nil
        for _, item in ipairs(self.npcHandler.shopItems) do
            if(item.id == itemid and item.subType == subType) then
                shopItem = item
                break
            end
        end

        if(shopItem == nil) then
            print('[Warning - ' .. getCreatureName(getNpcId()) .. '] NpcSystem:', 'ShopModule.onBuy - Item not found on shopItems list')
            return false
        end

        if(shopItem.buy == -1) then
            print('[Warning - ' .. getCreatureName(getNpcId()) .. '] NpcSystem:', 'ShopModule.onBuy - Attempt to purchase an item which only sellable')
            return false
        end

        if(amount <= 0) then
            print('[Warning - ' .. getCreatureName(getNpcId()) .. '] NpcSystem:', 'ShopModule.onBuy - Attempt to purchase ' .. amount .. ' items')
            return false
        end

        local subType, count = shopItem.subType or 0, amount
        local backpack, backpackPrice, totalCost = 1988, 20, amount * shopItem.buy
        if(inBackpacks) then
            totalCost = totalCost + (math.max(1, math.floor(count / getContainerCapById(backpack))) * backpackPrice)
        end

        local parseInfo = {
            [TAG_PLAYERNAME] = getPlayerName(cid),
            [TAG_ITEMCOUNT] = amount,
            [TAG_TOTALCOST] = totalCost,
            [TAG_ITEMNAME] = shopItem.name
        }
        if(getPlayerMoney(cid) < totalCost) then
            local msg = self.npcHandler:getMessage(MESSAGE_NEEDMONEY)
            doPlayerSendCancel(cid, self.npcHandler:parseMessage(msg, parseInfo))
            return false
        end

        local a, b = doNpcSellItem(cid, itemid, count, subType, ignoreCap, inBackpacks, backpack)
        if(a < amount) then
            local msgId = MESSAGE_NEEDMORESPACE
            if(a == 0) then
                msgId = MESSAGE_NEEDSPACE
            end

            local msg = self.npcHandler:getMessage(msgId)
            parseInfo[TAG_ITEMCOUNT] = a

            doPlayerSendCancel(cid, self.npcHandler:parseMessage(msg, parseInfo))
            if(NPCHANDLER_CONVBEHAVIOR ~= CONVERSATION_DEFAULT) then
                self.npcHandler.talkStart[cid] = os.time()
            else
                self.npcHandler.talkStart = os.time()
            end

            if(a > 0) then
                doPlayerRemoveMoney(cid, ((a * shopItem.buy) + (b * backpackPrice)))
                return true
            end

            return false
        end

        local msg = self.npcHandler:getMessage(MESSAGE_BOUGHT)
        doPlayerSendTextMessage(cid, MESSAGE_INFO_DESCR, self.npcHandler:parseMessage(msg, parseInfo))

        doPlayerRemoveMoney(cid, totalCost)
        if(NPCHANDLER_CONVBEHAVIOR ~= CONVERSATION_DEFAULT) then
            self.npcHandler.talkStart[cid] = os.time()
        else
            self.npcHandler.talkStart = os.time()
        end

        return true
    end

I guess the boolean is a, being amount the number, but can't understand why this only happens when buying too fast.