Casting string to number doesn't work!

programming flash
1

[SIZE=3][FONT=Calibri]Hi,[/FONT][/SIZE]
[SIZE=3][FONT=Calibri]I have Input text field on Scene with instance name number_txt.[/FONT][/SIZE]
[SIZE=3][FONT=Calibri]This example throws NaN in otput window.[/FONT][/SIZE]
[SIZE=3][FONT=Calibri]Why?[/FONT][/SIZE]
[SIZE=3][FONT=Calibri]Thanks[/FONT][/SIZE]

 
this.onMouseDown=function()
{
 trace(Number(number_txt.text));
}