[lazarus] Is NIL constant?

[DMo2]

I can't compile program like this
LINE 218:
    property Bitmap: TBitmap read FBitmap write SetBitmap default nil;

(218,67) Error: Property can't have a default value
(218,70) Error: The default value of a property must be constant

I suppose that declaration of  NIL is different

I can remove this, but I want know why?


Darek