Unable to cast Instance to CoordinateFrame?

Hello everybody, as again, i'm making a doors game and i have encountered an error.

This is the error:

14:00:25.785 Unable to cast Instance to CoordinateFrame - Server - Monster:14 14:00:25.785 Stack Begin - Studio 14:00:25.785 Script 'ServerScriptService.Server.Monster', Line 14 - Studio - Monster:14 14:00:25.785 Stack End - Studio

This is the code i believe responsible.

function monsterModule.lerpTo(m, targ)
    local alp = 0
    local speed = 50
    local dist = (m.PrimaryPart.Position - targ.Position).Magnitude
    local relativeSpeed = dist / speed
    local startC = m.PrimaryPart.CFrame
    local loop = nil
    local reachedTarg = Instance.new('BindableEvent')

    loop = monsterModule.runServ.Heartbeat:Connect(function(del)
        local goalC = startC:Lerp(targ, alp)
        m:PivotTo(goalC)
        alp += del / relativeSpeed
        if alp >= 1 then
            loop:Disconnect()
            reachedTarg:Fire()
        end
    end)

    reachedTarg.Event:Wait()

end

If you wish to answer, please do! Any help is appreciated!