undefined! really !?

var t;
t = 'really ' + t;
trace( t );

I stumped over this ActionScript easter egg while doing some XML parsing for nodes that some times just weren't there. And as the zealot that I am I thought that casting everything including Strings would be a good idea. It wasn't.
Insert rant about Player 6 vs Player 7 handling of this matter in conjunction with the ECMA-262 compliance.