无效的参数 #-1 传递给 'addBody' 函数 (应传递代理对象，但传递了 nil)。

我正在使用这段代码来生成对象，但有时会出现这个错误（Bad argument #-1 to 'addBody' (Proxy expected, got nil)），你有什么想法吗？

local numberRock = 1

local function clearRock( thisRock )
   display.remove( thisRock ) ; thisRock = nil
end

local function spawnRocks()
    rockChek = math.random(1, 7)
    if  rockChek == 1 then
        rocks = display.newImageRect("rockes/rock1.png",110 ,86 )
    elseif  rockChek == 2 then
        rocks = display.newImageRect("rockes/rock2.png",97 ,65 )
    elseif  rockChek == 3 then
        rocks = display.newImageRect("rockes/rock3.png",101 ,61 )
    elseif  rockChek == 6 then
        rocks = display.newImageRect("rockes/rock4.png",95 ,59 )
    elseif  rockChek == 7 then
        rocks = display.newImageRect("rockes/rock5.png",107 ,77 )
    end
    for i=1,numberRock do
        rocks.x = math.random(-10, 400);
        rocks.y =display.contentHeight + 100;
        transition.to( rocks, { time=2000,  y= display.contentHeight - display.contentHeight - 100 , onComplete=clearRock  } );
        physics.addBody( rocks, "dynamic", {density=.1, bounce=0.1, friction=.2, shape= shape2 ,filter=playerCollisionFilter } )
        rocks.name = "rocks"
        rocks.isSensor = true
        rocks.rotation =  math.random(100, 1800)
   end
end
Rockspawner1 = timer.performWithDelay( 1500, spawnRocks, 0 )