how to add wms to QGis via cpp-plugin

Question

I try to add a wms-Layer to QGIS Desktop from my plugin(c++)

QString uri= "crs=EPSG:28992&layers=natura2000:natura2000&styles=&format=image/png&url=http://geodata.nationaalgeoregister.nl/natura2000/ows?

QString myProviderName      = "wms";
QgsRasterLayer* lyr= mQGisIface->addRasterLayer(uri ,"test", myProviderName);

But in QGis i get the Message, the extent of the layer could not be calculated because of unknown dataprovider.

What do i wrong? In QGIs Desktop manually adding this wms-layer is possible without problems. I also tested several other wms with same result.

Thanks for all help.