jpl.JRef usage

I tried to call a method of an object from JPL ("k" would print out
some text, just for testing):

Kiiro k = new Kiiro();
new Query(":- use_module(library(jpl)).");
jpl.JRef jref = new JRef(k);
q1 = new Query("jpl_call("+jref+",kiir,[],_).");


error message:

Exception in thread "main" jpl.PrologException: PrologException:
error(syntax_error(operator_expected),
string('jpl_call(Kiiro@19b49e6,kiir,[],_). . ', 14))
at jpl.Query.get1(Query.java:338)
at jpl.Query.getSubstWithNameVars(Query.java:388)
at jpl.Util.textToTerm(Util.java:163)
at jpl.Query.<init>(Query.java:198)
at Prologtojava01.main(Prologtojava01.java:26)


I'm aware, that JRef is for some reason deprecated, but I don't know
why, and what should be used instead (the release notes didn't mention
any changes related to this).

My JPL version is:
6 ?- [jpl_versions_demo].

prolog library version: 3.1.3-alpha
java library version: 3.1.3-alpha
c library version: 3.1.3-alpha

(It came with the latest stable swi release.)
.