QFileDialog::getSaveFileName Invalid URL warning

I develop the VapourSynth Editor.
https://bitbucket.org/mystery_keeper/vapoursynth-editor

Lately two users have reported that invoking file save dialog gives them this warning:
Qt warning: Invalid URL: QUrl("1080p.vpy - 44757.png")
The same happens when they change the file name in that dialog.
http://forum.doom9.org/showthread.php?p=1783156#post1783156

It doesn't happen on my system:
Windows 8.1 Pro, Qt 5.7.0 MSVC2015, Qt 5.5.0 GCC 5.2.
But I'm concerned with my users' experience.

template<> inline char *toString(const QUrl &uri)

{ if (!uri.isValid()) return qstrdup(qPrintable(QStringLiteral("Invalid URL: ") + uri.errorString())); return qstrdup(uri.toEncoded().constData()); }

In "qtest.h". Looks like it could be the culprit.

The way to reproduce:
In Linux write a program that installs a Qt messages handling function with qInstallMessageHandler();
Invoke QFileDialog::getSaveFileName();
Try typing the file name and watch your message handler spit out warnings.