Set QT_SYSROOT property

[Mark81]

I'm trying to change the value of QT_SYSROOT because I moved the Qt5 installation.
According to the documentation I should be able to do that.

But here what happens:

$ ./qmake -query
QT_SYSROOT:
QT_INSTALL_PREFIX:/mnt/dev/qt5
QT_INSTALL_ARCHDATA:/mnt/dev/qt5
QT_INSTALL_DATA:/mnt/dev/qt5
...

$ ./qmake -set QT_SYSROOT /mnt/dev/sysroot

$ ./qmake -query
QT_SYSROOT:/mnt/dev/sysroot
QT_SYSROOT:
QT_INSTALL_PREFIX:/mnt/dev/qt5
QT_INSTALL_ARCHDATA:/mnt/dev/qt5
...

$ ./qmake -query QT_SYSROOT

What am I doing wrong?