Problem with stringifying a JSON string consisting of Chinese characters

[MAC420965]

The following code fails in a Delphi 11.1/Windows 64 Bit application with the conditional JSON_UTF8 defined in the Neslib.Json.* units:

program jsonwriter_err;

{$APPTYPE CONSOLE}

{$R *.res}

uses
System.SysUtils, Neslib.Json;

var
doc: IJsonDocument;
s1, s2: utf8string;

begin
s1 := '{"s":"クランクケース/全面カバー"}';
doc := TJsonDocument.Parse(s1);
s2 := doc.Root.ToJson(false);
WriteLn(Utf8ToString(s2));
ReadLn;
end.

Call stack:

:00007FFD1609CD29 ; C:\WINDOWS\System32\KERNELBASE.dll
System._RaiseAtExcept(???,???)
System.SysUtils.AssertErrorHandler('Assertion fehlgeschlagen','... Neslib.Json.IO.pas',1586,$712F5D)
System._Assert(???,???,???)
Neslib.Json.IO.TJsonWriter.WriteEscapedString('クランクケース/全面カバー')
Neslib.Json.IO.TJsonWriter.WriteString('クランクケース/全面カバー')
Neslib.Json.TJsonValue.WriteTo(TJsonWriter($258AEED80A0) as IJsonWriter)
Neslib.Json.TJsonValue.TJsonDictionary.WriteTo(TJsonWriter($258AEED80A0) as IJsonWriter)
Neslib.Json.TJsonValue.WriteTo(TJsonWriter($258AEED80A0) as IJsonWriter)
Neslib.Json.TJsonValue.ToJson(False)