FindBugs ant UnionBugs2 error help

[DAVID RYAN TOLLEY]

I need help figuring out why when I run a UnionBugs2 target, I am getting an error even though the combined report is being generated. The following is the error I am receiving:

fbcombinereports:
Executing findbugs from ant task
unioning bugs...
Trouble reading/parsing /Users/dtolley/blt/app/main/core/build/Bug-Report-monitoringlib.xml
Trouble reading/parsing /Users/dtolley/blt/app/main/core/build/Bug-Report-test-support-common.xml
Trouble reading/parsing /Users/dtolley/blt/app/main/core/build/Bug-Report-test-support-strictunit.xml
ATL MAIN.fbcombinereports: 2201 ms

BUILD FAILED
java.lang.AbstractMethodError: edu.umd.cs.findbugs.anttask.AbstractFindBugsTask.afterExecuteJavaProcess(I)V
at edu.umd.cs.findbugs.anttask.AbstractFindBugsTask.execFindbugs(AbstractFindBugsTask.java:338)
at edu.umd.cs.findbugs.anttask.AbstractFindBugsTask.execute(AbstractFindBugsTask.java:207)
at org.apache.tools.ant.UnknownElement.execute(UnknownElement.java:288)
at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.apache.tools.ant.dispatch.DispatchUtils.execute(DispatchUtils.java:106)
at org.apache.tools.ant.Task.perform(Task.java:348)
at org.apache.tools.ant.Target.execute(Target.java:357)
at org.apache.tools.ant.Target.performTasks(Target.java:385)
at org.apache.tools.ant.Project.executeSortedTargets(Project.java:1324)
at org.apache.tools.ant.Project.executeTarget(Project.java:1293)
at org.apache.tools.ant.helper.DefaultExecutor.executeTargets(DefaultExecutor.java:41)
at org.apache.tools.ant.Project.executeTargets(Project.java:1176)
at org.apache.tools.ant.Main.runBuild(Main.java:758)
at org.apache.tools.ant.Main.startAnt(Main.java:217)
at org.apache.tools.ant.Main.start(Main.java:179)
at org.apache.tools.ant.Main.main(Main.java:268)

And here is the ant target I created for it:

<target name="fbcombinereports">
<taskdef name="UnionBugs2" classname="edu.umd.cs.findbugs.anttask.UnionBugs2" classpath="${findbugs.home}/lib/findbugs-ant.jar">
</taskdef>

<UnionBugs2 to="Bug-Report.xml" home="${findbugs.home}">
<fileset dir="">
<include name="Bug-Report-*.xml">
</include>
</fileset>
</UnionBugs2>
</target>

Thank you, hopefully I can resolve this soon with your help!