String access error in class

I have included a stripped down version of a class I am using to store 
configuration data in an ini file.

This works when I am using it from a simple console app. But when I 
include it in a larger project, the constructor throws this error:

'Access violation at address 004050FC in module 'app.exe'. Write of 
address 00000008'.

This happens on the line:  ComputerName:='test';

Obviously 00000008 is not a valid address. But I thought delphi handles 
string address automatically.

What have I done wrong?

Thanks.



unit Configuration;

interface

uses windows,inifiles,sysutils;

type
TConfig = Class(TObject)
     public
       FileName:string;

       ComputerName:String;
       //...

       constructor Create(); overload;
       constructor Create(FN:String); overload;

       //procedure saveFile(); overload;
       //procedure saveFile(FN:string); overload;

       //function checkErrors():string;
       //procedure fixErrors();


   end;


var

   IniFile : TIniFile;

implementation

constructor TConfig.Create(Fn:string)overload;
begin

   IniFile := TIniFile.Create(Fn);
   ComputerName:='test';

   filename:=fn;
   //savefile(fn);

end;

constructor TConfig.Create()overload;
begin
   Create(Paramstr(0)+'.ini');
end;

end.