Creating QJsonDocument from json returns null

[rogorido]

I'm trying to create a QJsonDocument (see minimal example below) and I'm getting all the time a void QJsonDocument. If I run the small program I get:
is null: "no error occurred"

I'm using Qt 5.9.3 (in archlinux). Any ideas?

#include <QCoreApplication>
#include <QDebug>
#include <QJsonObject>
#include <QJsonDocument>
#include <QJsonParseError>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    QString json_str;
    QJsonDocument json_doc;
    QJsonParseError error_json;

    json_str = "{\"country\": \"Germany\", \"population\": 100 }";

    json_doc.fromJson(json_str.toUtf8(), &error_json);

    if (json_doc.isNull())
        qDebug() << "is null: " << error_json.errorString();
    else
        qDebug() << "is not null";

    return a.exec();
}

[VRonin]

fromJson is static and returns the parsed document http://doc.qt.io/qt-5/qjsondocument.html#fromJson

json_doc=QJsonDocument::fromJson(json_str.toUtf8(), &error_json);

[rogorido]

you're right @VRonin. It's working now. THanks a lot. Sorry for the noise.