abstract method with an optinally parameter

Hi @all

I have a problem with an optinally parameter in abstract method.

Type TSay
	Method Text(t:String = "everybody") Abstract
End Type



Type TSay_hi Extends TSay
	Method Text(t:String = "everybody")
		Print "Hi "+t
	End Method 
End Type 


Type TSay_hello Extends TSay
	Method Text(t:String = "everybody")
		Print "Hello "+t
	End Method 
End Type 


Global Say:TSay = New TSay_hi

Say.text("simi")		'that works

Say.text()				'that doesn't work

What's wrong?
Or isn't that allowed?

Thanks
simi