Nullpointer exception when DEFINITIONS file passed as B machine

We obtain the following Exception (rather than a syntax error) when parsing an definition file instead of a machine:

$ probcli LibraryIO.def
! ### Unknown file extension, assuming B machine:
! def
! An error occurred !
! source(bmachine)
! Error while parsing machine:
! java.lang.NullPointerException
at java.util.TreeMap.compare(TreeMap.java:1188)
at java.util.TreeMap.put(TreeMap.java:531)
at de.be4.classicalb.core.parser.analysis.prolog.RecursiveMachineLoader.recursivlyLoadMachine(RecursiveMachineLoader.java:211)
at de.be4.classicalb.core.parser.analysis.prolog.RecursiveMachineLoader.loadAllMachines(RecursiveMachineLoader.java:84)
at de.be4.classicalb.core.parser.BParser.printASTasProlog(BParser.java:94)
at de.be4.classicalb.core.parser.BParser.fullParsing(BParser.java:446)
at de.prob.cliparser.CliBParser.doFileParsing(CliBParser.java:132)
at de.prob.cliparser.CliBParser.main(CliBParser.java:116)