FindBugs

When I try to run the Ant task, I get:

[findbugs] Running FindBugs...
[findbugs] java.lang.IllegalArgumentException: Unknown option:
-sortByClass
[findbugs] at
edu.umd.cs.findbugs.config.CommandLine.parse(CommandLine.java:259)
[findbugs] at
edu.umd.cs.findbugs.config.CommandLine.parse(CommandLine.java:206)
[findbugs] at edu.umd.cs.findbugs.gui2.Driver.main(Driver.java:54)
[findbugs] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native
Method)
[findbugs] at
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:3
9)
[findbugs] at
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImp
l.java:25)
[findbugs] at java.lang.reflect.Method.invoke(Method.java:597)
[findbugs] at
edu.umd.cs.findbugs.LaunchAppropriateUI.launch(LaunchAppropriateUI.java:127
)
[findbugs] at
edu.umd.cs.findbugs.LaunchAppropriateUI.main(LaunchAppropriateUI.java:203)
[findbugs] edu.umd.cs.findbugs.gui2.Driver [options] [project or analysis
results file]

My build.xml invokes FindBugs with:

<findbugs home="${findbugs.home}" output="html"
outputFile="findbugs_unit_tests/findbugs_output.html" jvmargs="-Xmx1024m"
reportLevel="high" failonerror="true">

It looks like FindBugs is trying to start the GUI rather than the command
line interface, which seems a bit weird.

System details:
Windows xp
Sun Java 1.6.0_06
Ant 1.7.0
FindBugs 1.3.7