[fpc-pascal] Range check error while evaluating constants

[ik]

Hello,

I have the following function:

function OneToTwoComplement(AValue: QWord): QWord;
begin
  Result := (AValue xor $FFFFFFFFFFFFFFFF) + 1;
end;

What am I missing here that makes the compiler complain about "Range
check error while evaluating constants" ?
Why doesn't it complain on :

function OneToTwoComplement(AValue: Int64): Int64;
begin
  Result := (Abs(AValue) xor $FFFFFFFFFFFFFFFF) + 1;
end;


Thanks,
Ido