ActionScript 3.0 :: High Score Table Setup - If Statement With Variable

I am trying to get a high score table set up and I have it all working except for an if statement which decides whether to put a zero in the time or not.
ActionScript Code:
variable declared at the top
var gap:String;

This is getting data that is stored - it works fine except for the underlined "gap" - The "gap" is meant to be calling the variable which has the below if statement
ScoreInputScore.text=String(SpotTheDifferenceGame._minute)+gap+String(SpotTheDifferenceGame._second);
if (SpotTheDifferenceGame._second<10) {
gap=(":0");
} else {
gap=(":");
}

I thought by have the variable called in in the function and having the if statement with gap= it would do the specified text in the dynamic text box. When I put a trace on "gap" it traces with the correct text depending on what the number is but in my dynamic text box it displays as number null number.
// displays like this when trace put on it
:0
// displays like this in dynamic text box
2null02