Model SetPrimaryPart because no PrimaryPart has been set error?

Model:SetPrimaryCFrame() failed because no PrimaryPart has been set, or the PrimaryPart no longer exists. Please set Model.PrimaryPart before using this.

Gun.Parent=game.Workspace

RunS.RenderStepped:connect(function()
Gun:SetPrimaryPartCFrame(Cam.CFrame*CFrame.new(0,-1,2))
end)

I SET THE PRIMARY PART OF THE GUN ALREADY AND SET THE PRIMARY PART IN THE SCRIPT ALSO? Why do I keep getting this error???