尝试索引全局变量 'object' （一个空值）。

这是我一直在遇到的错误：

Game.lua:66: attempt to index global 'Spears' (a nil value)
stack traceback:
                         Game.lua:66: in function '_listener'

下面是一些代码，显示错误发生的位置：

    local Spears = {}
    local SpearsTimer
    local SpearsCounter = 1

    local delayTimer
    local removeListeners

end
end

local field = display.newCircle( 0, 0, 0 ) ; field.alpha = 0.3
field.name = "field"
field.x = display.contentCenterX ; field.y = display.contentCenterY
physics.addBody( field, "static", { isSensor=true, radius=320 } )

    local spawnSpears = function()
        local Fall = math.random(display.contentWidth * -0.2, display.contentWidth * 1.2)
        Spears[SpearsCounter] = display.newImageRect( "Spear.png", 15, 50 )
        Spears[SpearsCounter].x = Fall
        Spears[SpearsCounter].y = -200
        physics.addBody( Spears[SpearsCounter], "dynamic", {bounce = 0} )
        --Spears[SpearsCounter].collision = onCollision
        --Spears[SpearsCounter]:addEventListener( "collision", Spears[SpearsCounter] )
        transition.to( Spears[SpearsCounter], { rotation = Spears[SpearsCounter].rotation+720, time=15000, onComplete=spinImage } )
        Spears[SpearsCounter].gravityScale = 0.5
        sceneGroup:insert(Spears[SpearsCounter])
        group:insert(Spears[SpearsCounter])
        SpearsCounter = SpearsCounter + 1
    end

   SpearsTimer = timer.performWithDelay( 5000, spawnSpears, -1 )
错误指向第 66 行，这是这行代码：

        Spears[SpearsCounter] = display.newImageRect( "Spear.png", 15, 50 )

那我做错了什么呢？

哦，记住我正在尝试使对象在屏幕上随机生成并跌落/到屏幕的中心。我放置了径向重力。