local global variable returns nil, bug?

Declared in another script:

_G.Stamina = 100

The script that uses it and says "comparing number to nil"

local SBar = script.Parent:WaitForChild("BackG"):WaitForChild("SBar")
while _G.Stamina > 0 and wait(.1) do
    print "test"
    SBar:TweenSize(UDim2.new(0, _G.Stamina/100, 0, 1, 0), "Out", "Quint", .1, true)
end

while true and wait(.1) do
    wait(.1)
    local print = "s"
end

Is this a bug or am I doing something wrong here?