Cannot call ToString successfully

[chuacw]

The following code throws an exception:

uses System.SysUtils, Neslib.MultiPrecision;
var
  LDD: DoubleDouble;
  LValue: string;
begin
   LDD := DoubleDouble(Double.MaxValue);
   LDD := LDD+1;
   LValue := LDD.ToString(TMPFloatFormat.Fixed);
end;