尝试调用全局变量'gameover'（一个表值）

我正在使用onCollision事件制作一个小游戏：

local function onCollision(event)
    if event.phase == "began" and gameIsActive == true then
        local obj1 = event.object1;
        local obj2 = event.object2;

        if obj1.name == "bill" then
        if     obj2.name == "rocks" then gameover()
        elseif obj2.name == ""   then
        end
    end
    end
end
Runtime:addEventListener( "collision", onCollision )

但是我有一个问题，代码在第一次运行时按预期工作，但是如果重新启动游戏，我会收到以下错误：

File: game.lua
Line: 649
Attempt to call global 'gameover' (a table value)

stack traceback: game.lua:649: in function <game.lua:643> ?: in function <?:221>

649 = if obj2.name == "rocks" then gameover()
643 = local function onCollision(event)

你有什么想法吗？

gameover函数：

function gameover()

rightTAP:removeEventListener("tap", movePLAYERr)
leftTAP:removeEventListener("tap", movePLAYERl)

timer.pause(spawnBits)
timer.pause(Rockspawner1)
timer.pause(tmrscore)
timer.pause(updateScoretimer)
timer.pause(spawnDinosControll)

audio.pause( drillingChannel )

drillbg.isVisible = false
scoreText.isVisible = false

Restartg = display.newText( "Restart", 0, 0,nil, 20)
Restartg:setFillColor(0, 0, 0)
screenGroup:insert(Restartg)
Restartg.x= display.contentWidth/2
Restartg.y= display.contentHeight/2 + 160

Restartg:addEventListener("tap", RestartGame1)

end