Corona SDK(LUA) - attempt to call upvalue 'spawnEnemy'(a nil value)

Corona SDK(LUA) - attempt to call upvalue 'spawnEnemy'(a nil value)

I just trying to add eventListener to a object, which should disappear
when I tap on it. But I get error mentioned in the title. Here is my whole
code at this point :
-- housekeeping stuff
display.setStatusBar(display.HiddenStatusBar)
local centerX = display.contentCenterX
local centerY = display.contentCenterY
-- set up forward references
local spawnEnemy
-- preload audio
-- create play screens
local function createPlayScreen()
local bg = display.newImage("background.png")
bg.y = 130
bg.x = 100
bg.alpha = 0
local planet = display.newImage("planet.png")
planet.x = centerX
planet.y = display.contentHeight +60
planet.alpha = 0
transition.to( bg, { time = 2000, alpha = 1, y = centerY, x =
centerX } )
local function showTitle()
local gametitle = display.newImage("gametitle.png")
gametitle.alpha = 0
gametitle:scale (4, 4)
transition.to( gametitle, { time = 500, alpha = 1, xScale = 1,
yScale = 1 })
spawnEnemy()
end
transition.to( planet, { time = 2000, alpha = 1, y = centerY,
onComplete = showTitle } )
end
-- game functions
local function shipSmash(event)
local obj = event.target
display.remove( obj )
end
local function spawnEnemy()
local enemy = display.newImage("beetleship.png")
enemy.x = math.random(20, display.contentWidth - 20)
enemy.y = math.random(20, display.contentHeight - 20)
enemy:addEventListener ( "tap", shipSmash )
end
local function startGame()
end
local function planetDamage()
end
local function hitPlanet(obj)
end
createPlayScreen()
startGame()
And here is how error window looks like :

I'm kinda new in this area(LUA programming) so sorry for maybe dumb syntax
error or something, but what I saw is that this error shows up after I
write this line of code: enemy:addEventListener ( "tap", shipSmash )