Custom TXmlIniFile method creates an error - delphi

Question

I have created this unit containing a class derived from TXmlIniFile:
unit PAXMLIniFile;
interface
uses
System.SysUtils,
System.IniFiles,
Xml.XmlDoc,
Xml.XMLIntf,
Xml.XMLIniFile;
type
TMyStandaloneXmlIniFile = class(TXmlIniFile)
strict private
FDocument: IXMLDocument;
FFileName: string;
public
constructor Create(const AFileName: string);
procedure UpdateFile; override;
end;
implementation
constructor TMyStandaloneXmlIniFile.Create(const AFileName: string);
begin
if FileExists(AFileName) then
FDocument := LoadXMLDocument(AFileName)
else
begin
FDocument := NewXMLDocument;
FDocument.Options := FDocument.Options + [doNodeAutoIndent];
end;
if FDocument.DocumentElement = nil then
FDocument.DocumentElement := FDocument.CreateNode('Sections');
TCustomIniFile(Self).Create(AFileName);
FFileName := AFileName;
inherited Create(FDocument.DocumentElement);
end;
procedure TMyStandaloneXmlIniFile.UpdateFile;
begin
FDocument.SaveToFile(FFileName);
end;
end.
Now whenever I call the UpdateFile method I get an error message "Wrong parameter". What is wrong here?
Here is the test program:
program BonkersTest;
{$APPTYPE CONSOLE}
{$R *.res}
uses
System.SysUtils, PAXMLIniFile, Winapi.ActiveX;
var
MyStandaloneXmlIniFile: TMyStandaloneXmlIniFile;
MyXmlFile: string;
begin
try
CoInitialize(nil);
MyXmlFile := ChangeFileExt(ParamStr(0), '.xml');
MyStandaloneXmlIniFile := TMyStandaloneXmlIniFile.Create(MyXmlFile);
try
MyStandaloneXmlIniFile.WriteString('Section1', 'Ident1', 'Value1');
MyStandaloneXmlIniFile.UpdateFile;
finally
MyStandaloneXmlIniFile.Free;
end;
CoUninitialize;
except
on E: Exception do
Writeln(E.ClassName, ': ', E.Message);
end;
end.