-Dpicocli.trace (without value) throws exception when used with Groovy

[remkop]

$ C:/apps/groovy-2.4.5/bin/groovy -Dpicocli.trace /cygdrive/c/Users/remko/IdeaProjects/pico-cli/src/test/groovy/picocli/groovy/checksum.groovy *.jar --help
Caught: java.lang.IllegalArgumentException: No enum constant picocli.CommandLine.TraceLevel.true
java.lang.IllegalArgumentException: No enum constant picocli.CommandLine.TraceLevel.true
        at picocli.CommandLine$TraceLevel.valueOf(CommandLine.java:4525)
        at picocli.CommandLine$TraceLevel.lookup(CommandLine.java:4531)
        at picocli.CommandLine$Tracer.<init>(CommandLine.java:4534)
        at picocli.CommandLine$Tracer.<init>(CommandLine.java:4533)
        at picocli.CommandLine.<init>(CommandLine.java:134)
        at picocli.groovy.PicocliBaseScript.createCommandLine(PicocliBaseScript.java:177)
        at picocli.groovy.PicocliBaseScript.getOrCreateCommandLine(PicocliBaseScript.java:159)
        at picocli.groovy.PicocliBaseScript.run(PicocliBaseScript.java:104)