Why can't I add an on(change) handler to my comboBox!?!

This is really driving me mad. I place a comboBox on the stage, select it, give it an instance name, then add the following actionscript:
Code:
on(change){
   trace("hello");
}
... but when I try to run it I get the following errors:
Scene=Scene 1, Layer=Layer 14, Frame=1: Line 14: Invalid mouse event specified.
on(change){

Scene=Scene 1, Layer=Layer 14, Frame=1: Line 15: Statement must appear within on/onClipEvent handler
trace("hello");

Scene=Scene 1, Layer=Layer 14, Frame=1: Line 16: Unexpected '}' encountered
}
What am I doing wrong? This should be so simple...