Attempt to compare nil with number Error in Lua (Corona Lab) -

i'm new corona (lua). after running game, game seems work perfectly, until few seconds later when following error: 'attempt compare nil number'

local function gameloop()

-- create new asteroids createasteroid()  -- remove asteroids have been drifted off screen = #asteroidstable, 1, -1     local thisasteroid = asteroidstable [i]      if (thisasteroid.x < -100 or         thisasteroid.x > display.contentwidth  + 100 or         thisasteroid.y < -100 or         thisasteroid.y > display.contentheight + 100 )                display.remove( thisasteroid )         table.remove( asteroidstable)      end  end 

end

---

as can see above, 'thisasteroid' in 'asteroidstable = {}' defined variable in top of module , outside of function.

local asteroidstable = { }

thanks help!

either thisasteroid.x, thisasteroid.y, display.contentwidth or display.contentheight nil.

use print(thisasteroid.x) etc find out 1 nil.

you should line number with error message helps find problem.

once have found nil value either have prevent becoming nil or if can't should restrict comparison non-nil values.