.LUA: Trouble with nil value

[Organized_Chaos]

Hello everyone.. As you can probably tell I am new at LUA programing. I followed all the directions in this tutorial at http://www.evilmana.com/tutorials/lua_tutorial_03.php and I am having problems. How do I make this error go away and make my "program" work?
I get an error message that says
"Error: Variable Tut.LUA:18: attempt to perform arithmetic on global 'myBirthYear' (a nil value)
Press start to restart"
("Variable Tut.LUA" is my file name.)
-
Here is my code:

-- NOTES
-- Green Color Object
green = Color.new(0, 255, 0)

-- Store birth year in variable myBirthYear
myBirthyear = 1991

-- Store current year in currentYear
currentYear = 2008

-- create an empty variable
myAge = nil

-- Store some text in a variable (Note the 

space after the last word before the end ")
someText = "My age is roughly "

-- Subtract myBirthYear from currentYear 

and store in myAge
myAge = currentYear - myBirthYear

-- Print our text variable to the screen 

(NOTE that there are no "" surrounding 

someText)
screen:print(10,100,someText .. 

myAge,green)

-- Buffer offscreen to onscreen (If this 

was a human, offscreen would be thinking 

about it, onscreen would be saying it)
screen.flip()
-- Loop forever
while true do
screen.waitVblankStart()
end

The only thing I did differently was use my birth year and the current year instead of the years in this tutorial. And I added a few custom notes using the "-- text" format. Since this was a tutorial I figured I might add in a few things in case I use this file later as a reference or something. That stuff shouldn't make a difference should it?