[fpc-pascal] why dynamic array created in constructor not automatically free in destructor

[Dennis Poon]

I have a class like;
------------
TMyClass=class
public
   ar : array of integer;
   constructor Create;
  destructor destroy;override;
end;

TMyClass.Create;
begin
    inherited;
    SetLength(ar, 100);
end;

TMyClass.Destroy;
begin
    ar := nil;//<--- this is needed otherwise a memory leak is reported!
   inherited;
end;
----------

I would expect the compiler would automatically insert this ar := nil on 
my behalf because it seems like it does it for strings.
Am I missing some compiler directives?

Dennis