Silent failure on node edit/update

Hi,

I encountered the following problem:
* created a particular content type
* mapped the type and fields to RDF classes and properties (using an own vocabulary)
* created a couple of nodes of that content type
* everything worked great: RDF is generated well, SPARQL endpoint works
* then, I tried editing an existing node that has RDF mappings and I stumbled upon a silent PHP failure:

PHP Fatal error: Unsupported operand types in /var/www/.../sites/all/modules/rdfx/rdfx.module on line 224, referer: http://.../node/2/edit?render=overlay

Any idea how I can fix it?

Thanks!