ServerScriptService.Script:3: attempt to index nil with 'IsKeyDown' ?

I encountered a problem in my script when i was checking if a player was holding down the Q key have no idea how i can fix this.

Here are my two scripts :

The Client script

local UserInputService = game:GetService("UserInputService")

local player = game.Players.LocalPlayer

local Mouse = player:GetMouse()

local character = player.Character or player.CharacterAdded:Wait()

local block = game.Workspace.Part:Clone()
block.Parent = workspace

local Key = 'Q'

local debounce = true

UserInputService.InputBegan:Connect(function(Input ,IsTyping)
    if IsTyping then return end
    local KeyPressed = Input.KeyCode
    if Input.UserInputType == Enum.UserInputType.Keyboard then
        if KeyPressed == Enum.KeyCode[Key]and debounce and character then
            game.ReplicatedStorage.FireBallEvent:FireServer(Mouse.Hit, UserInputService, Key)
        end
    end
end)

The Server Script

local fireball = Instance.new("Part", workspace)

game.ReplicatedStorage.FireBallEvent.OnServerEvent:Connect(function(player, mouse, UserInputService, Key)
    while wait(1) do 
        if UserInputService:IsKeyDown(Enum.KeyCode[Key]) then
            print("the fireball is being charged!")
            fireball.Size = fireball.Size + Vector3.new(0.01,0.01,0.01) 
        else
            print("the fireball has been fired!")
            break;
        end
    end
end)