Exception running findbugs ant task

I'm a new findbugs user. I've installed findbugs-1.1.2 and
successfully used the gui on my project to identify bugs. Now I'm
trying to add it to my ant build, and I get an error.

I've defined the task, and added a findbugs target

        <target name="findbugs">
                <findbugs home="${findbugs.home}"
                        output="xml"
                        outputFile="${findbugs.dir}/findbugs-report.xml" >
                <sourcePath path="${build.src.dir}" />
                <class location="${build.classes.dir}" />
                </findbugs>
        </target>

but when I run this target I get the following

Buildfile: build.xml

findbugs:
[findbugs] Running FindBugs...
[findbugs] Error: The findbugs.home property is not set!
[findbugs] Exception in thread "main" java.lang.ExceptionInInitializerError
[findbugs]     at
edu.umd.cs.findbugs.TextUICommandLine.<init>(TextUICommandLine.java:78)
[findbugs]     at edu.umd.cs.findbugs.FindBugs2.main(FindBugs2.java:681)
[findbugs] Caused by: java.lang.NullPointerException
[findbugs]     at
edu.umd.cs.findbugs.DetectorFactoryCollection.loadPlugins(DetectorFactoryCollection.java:250)
[findbugs]     at
edu.umd.cs.findbugs.DetectorFactoryCollection.ensureLoaded(DetectorFactoryCollection.java:214)
[findbugs]     at
edu.umd.cs.findbugs.DetectorFactoryCollection.instance(DetectorFactoryCollection.java:99)
[findbugs]     at
edu.umd.cs.findbugs.config.ProjectFilterSettings.<init>(ProjectFilterSettings.java:93)
[findbugs]     at
edu.umd.cs.findbugs.config.ProjectFilterSettings.createDefault(ProjectFilterSettings.java:110)
[findbugs]     at
edu.umd.cs.findbugs.config.UserPreferences.<init>(UserPreferences.java:75)
[findbugs]     at
edu.umd.cs.findbugs.config.UserPreferences.<clinit>(UserPreferences.java:72)
[findbugs]     ... 2 more
[findbugs] Output saved to dist/reports/findbugs/findbugs-report.xml

BUILD SUCCESSFUL
Total time: 0 seconds

The error says that findbugs.home is not set, but I'm sure it is. I've
used echoproperties to confirm, and if I change the value, then it
correctly reports an error that it cannot find findbugs.jar. So is
there something else that I'm forgetting to do? Is there anything else
I can try to figure this out? I'm running on Windows XP with
jdk1.5.0_10 and apache-ant-1.7.0.

Thanks for any help,

Alex Chapman