Number variable in text field

I am making a game in Flash and I’m trying to increment the score for each alien that’s killed in the game. There is a score variable that’s created on the main timeline:

var score:int;
score = 0;

Then, inside the alien movie clip, I want to record the score and give it a value each time the user hits the alien and then show the score in a dynamic text field called score_txt. So in the function that tracks the death of an alien, here’s what I have:

root.score_txt.text = root.score 

I keep getting error messages. Any clues?