Mouse Event Error please help

1

HelloI am tring a very simple Mouse event programme for some reason its giving me this error.I am creating two buttons and adding them to a sprite and then I am attaching addEvent listener to the sprite object and when any of the two buttons inside that sprite is clicked I am calling a function in the button class that returns the name of the button. but it is not calling this function and is tracing an undefined value here is the code.