Running queries via dbus

[Jacopo De Simoi]

Dear amarockers, 
  I've been investigating the reasons behind a large number of bug reports for krunner in which the interface was not providing search results anymore under some diverse conditions. 
One of these conditions relates to the "control audio player" runner, which performs collection queries on the amarok database via dbus. 
From the runner side it appears that the reply is never sent, so the runner is blocked for the usual dbus timeout (~25 secs) and this causes the issues reported in all these bug reports.
Running amarok with QDBUS_DEBUG=1 gives some interesting insight on what's going on:

here follows the interesting part:

QDBusConnectionPrivate(0x760f40) got message: QDBusMessage(type=MethodCall, service=":1.38015", path="/Collection", interface="org.kde.amarok.Collection", member="Query", signature="s", contents=("<query version="1.0"><limit value="10" /><filters><or><include field="title" value="in" /><or><include field="artist" value="in" /><or><include field="album" value="in" /></or></or></or></filters><includeCollection id="localCollection" /></query>") )                                                                                   
amarok:  XQR: include filter                                                                              
amarok:  XQR: include filter                                                                              
amarok:  XQR: include filter                                                                              
QDBusMarshaller: type `float' (135) is not registered with D-BUS. Use qDBusRegisterMetaType to register it
QDBusConnection: error: could not send reply message to service "": Marshalling failed: Unregistered type float passed in arguments                                                                                 
amarok:  sending async reply failed    

Can anybody help me figuring out what's causing this error? 
Thanks a lot

 __J