Catchable fatal error: Argument 1 passed to SMWSerializer::requireDeclaration() must be an instance of SMWExpResource, instance of SMWExpData given, called in /u/wikis/mediawiki-1.15.5/extensions/SemanticMediaWiki/includes/export/SMW_Serializer_RDFXML.php on line 158 and defined in /u/wikis/mediawiki-1.15.5/extensions/SemanticMediaWiki/includes/export/SMW_Serializer.php on line 218