Why can't I store OWL class in a Variable?

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I am not sure where exactly my problem or misunderstanding is. I am using OWL within Knowrob and Rosprolog.

In my OWL, hsr_objects:'Banana' is a subclass of hsr_objects:'Item'. So this behaves as expected:

?- owl_subclass_of(hsr_objects:'Banana', hsr_objects:'Item')
true.

But why does the error occur in the following prompt?

?- X=hsr_objects:'Banana', owl_subclass_of(X, hsr_objects:'Item')
Prolog query failed: error(type_error(rdf_object,hsr_objects:Banana),context(rdf_db:rdf_has/3,_434))