How to express argument of type QVariantMap in qdbus command line tool

Question

I'm trying to use the following command :

qdbus org.kde.krunner /org/kde/krunner org.kde.KDBusService.CommandLine

The signature is the following :

org.kde.KDBusService.CommandLine                -- int(QStringList arguments, QString working-dir, QVariantMap platform-data)

But I received the following error, and I don't know why :

qdbus org.kde.krunner /org/kde/krunner org.kde.KDBusService.CommandLine "ls" "/" "{}"
Sorry, can't pass arg of type 'QVariantMap'.

What can I do for casting the right format ?