What's wrong with this armory script?

Upgrade=function(Player, Ammo, Damage)
    if Player and Ammo and Damage then
        local BP,SG,BPGUN,SGGUN=Player:findFirstChild("Backpack"),Player:findFirstChild("StarterGear"),BP:findFirstChild("Gun"),SG:findFirstChild("Gun")
        repeat wait(.15) until BP and SG and BPGUN and SGGUN
        BPGUN["Stats"]["Ammo"].Value=Ammo
        SGGUN["Stats"]["Ammo"].Value=Ammo
        BPGUN["Stats"]["Damage"].Value=Damage
        SGGUN["Stats"]["Damage"].Value=Damage
    end
end

That's just the snippet of code that's erroring. It goes with an armory system with an onTouched function. Whenever I touch the brick, it's supposed to find my Backpack and StarterGear, but so far all I've gotten in the Output is Workspace.ArmoryControl:3: attempt to index global 'BP' (a nil value)

Why is it saying that? I've looked BP over and I can't seem to find why it can't find BP. How would I be able to fix it?

Thank you!