How to access a variable declared in a module?

in my game.lua file I have:

local sprites = require("sprites.lua")

sprites.lua contains

local iceberg = display.newImage("iceberg.png")
iceberg.x = _W/2
iceberg.y = _H/2
iceberg.alpha = 1

Now, I'd like to set "iceberg.alpha" to 0 from "game.lua", but If I try, Corona returns "attempt to index global iceberg (a nil value)"

Of course, sprites.lua contains

module(..., package.seeall)

What am I doing wrong?

I've even tried to use sprites.iceberg.alpha = 0, but obviously it doesn't work.

Thank you.