Error when saving as functional syntax

Trying to convert this file to functional syntax works with Protege but not with with OWLtools CLI

https://github.com/VirtualFlyBrain/VFB_owl/blob/master/src/owl/fbbt_vfb_ind_prc_nr.owl

owltools fbbt_vfb_ind_prc_nr.owl -o -f ofn vfb.ofn
=>
Feb 11, 2016 4:11:23 PM org.semanticweb.owlapi.util.CollectionFactory sortOptionally
WARNING: Misbehaving triple comparator, leaving triples unsorted: [ ... < long list of triples> ]
java.lang.IllegalArgumentException: Comparison method violates its general contract!
at java.util.ComparableTimSort.mergeLo(ComparableTimSort.java:740)
at java.util.ComparableTimSort.mergeAt(ComparableTimSort.java:477)
at java.util.ComparableTimSort.mergeCollapse(ComparableTimSort.java:402)
at java.util.ComparableTimSort.sort(ComparableTimSort.java:209)
at java.util.Arrays.sort(Arrays.java:1312)
at java.util.Arrays.sort(Arrays.java:1506)
at java.util.ArrayList.sort(ArrayList.java:1454)
at java.util.Collections.sort(Collections.java:141)
at org.semanticweb.owlapi.util.CollectionFactory.sortOptionally(CollectionFactory.java:111)
at org.coode.owlapi.functionalrenderer.OWLObjectRenderer.writeAxioms(OWLObjectRenderer.java:342)
at org.coode.owlapi.functionalrenderer.OWLObjectRenderer.visit(OWLObjectRenderer.java:265)
at uk.ac.manchester.cs.owl.owlapi.OWLOntologyImpl.accept(OWLOntologyImpl.java:1516)
at org.coode.owlapi.functionalrenderer.OWLFunctionalSyntaxOntologyStorer.storeOntology(OWLFunctionalSyntaxOntologyStorer.java:90)
at org.semanticweb.owlapi.util.AbstractOWLOntologyStorer.store(AbstractOWLOntologyStorer.java:182)
at org.semanticweb.owlapi.util.AbstractOWLOntologyStorer.storeOntology(AbstractOWLOntologyStorer.java:126)
at uk.ac.manchester.cs.owl.owlapi.OWLOntologyManagerImpl.saveOntology(OWLOntologyManagerImpl.java:1001)
at owltools.io.ParserWrapper.saveOWL(ParserWrapper.java:226)
at owltools.cli.CommandRunner.runSingleIteration(CommandRunner.java:3062)
at owltools.cli.CommandRunnerBase.run(CommandRunnerBase.java:76)
at owltools.cli.CommandRunnerBase.run(CommandRunnerBase.java:68)
at owltools.cli.CommandLineInterface.main(CommandLineInterface.java:12)

Reproduce the failure with the linked ontology and the shown owltools command, then inspect OWLObjectRenderer.writeAxioms and the OWL API sorting stack in the trace. Use ParserWrapper.java and CommandRunner.java as entry points; done means the conversion completes and writes vfb.ofn without the comparator exception.