ServerScriptService.MainScript:77: bad argument #2 to 'random' (interval is empty) ??

That's the whole script

-- Variables
DefaultMessageDelayTime = 4
LetterInterval = 0.05
MessageWaitSigns = {',', '?', '.'}
MessageWaitSignDelay = 1


MessageValue = game.ReplicatedStorage.Message
ImageValue = game.ReplicatedStorage.Message.SpeakerImage
SpeakerNameValue = game.ReplicatedStorage.Message.SpeakerName


-- Setup Player Images

PlayerImages = {}


PlayerService = game:GetService('Players')
function MakePlayerImage(Player)
    local Image = ''
    local Success,Error = pcall(function()
        local ID = PlayerService:GetUserIdFromNameAsync(Player.Name) or ''
        local PlayerImage, IsReady = PlayerService:GetUserThumbnailAsync(ID, Enum.ThumbnailType.AvatarBust, Enum.ThumbnailSize.Size420x420)
        if IsReady == true then
            -- Ready to use image
            Image = tostring(PlayerImage)
        else
            -- Use alternative picture
            Image = ("http://www.roblox.com/Thumbs/Avatar.ashx?x=100&y=100&Format=Png&username=%s"):format(Player.Name)
        end 
    end)
    if Success == true then
        -- Successful
    else
        Image = ("http://www.roblox.com/Thumbs/Avatar.ashx?x=100&y=100&Format=Png&username=%s"):format(Player.Name)
    end
    PlayerImages[Player.Name] = Image
end


game.Players.PlayerAdded:Connect(function(Player)
    MakePlayerImage(Player)
end)
for _, Player in pairs(game.Players:GetChildren()) do
    MakePlayerImage(Player)
end



-- Functions

function BroadcastMessage(Text, MessageDelayTime)

    for i = 1,#Text,1 do
        MessageValue.Value = string.sub(Text,1,i)
        for _, L in pairs(MessageWaitSigns) do
            if string.sub(Text,i,i) == L then
                wait(MessageWaitSignDelay)
            end
        end
        wait(LetterInterval)
    end
    if tonumber(MessageDelayTime) ~= nil then
        wait(MessageDelayTime)
    else 
        wait(DefaultMessageDelayTime) 
    end
    MessageValue.Value = 'None'
    return true
end


function SetMessageSpeaker(Name)
    if Name == 'Random' then
        local RandomPlayer = game.Players:GetChildren()[math.random(1,#game.Players:GetChildren())]
        Name = RandomPlayer.Name
    end
    ImageValue.Value = PlayerImages[Name] or '' 
    SpeakerNameValue = Name
end


















------------------------------ MAIN SCRIPT OF STORY ------------------------------------------------


SetMessageSpeaker('Random')

BroadcastMessage('Hello .... My name is Shebel .... I will be your guide!')