invalid argument #2 (Vector3 expected, got table) ???

I wanted to learn how to make fps framework but i got this error can someoen help me??

The script--

repeat
    wait()
until game:IsLoaded()

local plr = game.Players.LocalPlayer
local char = plr.Character
local mouse = plr:GetMouse()
local cam = workspace.CurrentCamera

local Run = game:GetService("RunService")

local RS = game.ReplicatedStorage
local GunModules = RS:WaitForChild("GunModules")
local GunModels = RS:WaitForChild("GunModels")

local Primary = "AK74"
local WeaponInHand
local WeaponData

local maincf = CFrame.new()

plr.CameraMode = Enum.CameraMode.LockFirstPerson

Run.RenderStepped:Connect(function()
    if WeaponInHand then
        WeaponInHand:SetPrimaryPartCFrame(
            cam.CFrame
            * maincf                -- The error is here
        )
    end
end)


function setup(weapon)
    WeaponData = require(GunModules:WaitForChild(weapon))

    maincf = WeaponData

    WeaponInHand = GunModels:WaitForChild(weapon)
    WeaponInHand.PrimaryPart = WeaponInHand:WaitForChild("Handle")
    WeaponInHand.Parent = cam
    for i, part in pairs(WeaponInHand:GetChildren()) do
        if part:IsA("BasePart") then
            part.Anchored = true
            part.CanCollide = false
        end
    end
end

setup(Primary)