ant generate fails with npe

[yunong]

The generate task, which generates the tarball for zookeeper libs, fails with:

api-xml:
  [javadoc] Generating Javadoc
  [javadoc] Javadoc execution
  [javadoc] Loading source files for package org.apache.zookeeper...
  [javadoc] Constructing Javadoc information...
  [javadoc] JDiff: doclet started ...
  [javadoc] /Users/yunong/workspace/yunong-zk/src/java/main/org/apache/zookeeper/server/auth/KerberosName.java:35: warning: Config is internal proprietary API and may be removed in a future release
  [javadoc] JDiff: writing the API to file '/Users/yunong/workspace/yunong-zk/src/java/lib/jdiff/zookeeper_3.4.3.xml'...
  [javadoc] import sun.security.krb5.Config;
  [javadoc]                         ^
  [javadoc] /Users/yunong/workspace/yunong-zk/src/java/main/org/apache/zookeeper/server/auth/KerberosName.java:36: warning: KrbException is internal proprietary API and may be removed in a future release
  [javadoc] import sun.security.krb5.KrbException;
  [javadoc]                         ^
  [javadoc] JDiff: finished (took 0s, not including scanning the source files).
  [javadoc] 2 warnings

BUILD FAILED
/Users/yunong/workspace/yunong-zk/build.xml:1578: java.lang.NullPointerException
    at org.apache.tools.ant.taskdefs.Javadoc.postProcessGeneratedJavadocs(Javadoc.java:2450)
    at org.apache.tools.ant.taskdefs.Javadoc.execute(Javadoc.java:1790)
    at org.apache.tools.ant.UnknownElement.execute(UnknownElement.java:292)
    at sun.reflect.GeneratedMethodAccessor4.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:601)
    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:435)
    at org.apache.tools.ant.Target.performTasks(Target.java:456)
    at org.apache.tools.ant.Project.executeSortedTargets(Project.java:1393)
    at org.apache.tools.ant.Project.executeTarget(Project.java:1364)
    at org.apache.tools.ant.helper.DefaultExecutor.executeTargets(DefaultExecutor.java:41)
    at org.apache.tools.ant.Project.executeTargets(Project.java:1248)
    at org.apache.tools.ant.Main.runBuild(Main.java:851)
    at org.apache.tools.ant.Main.startAnt(Main.java:235)
    at org.apache.tools.ant.launch.Launcher.run(Launcher.java:280)
    at org.apache.tools.ant.launch.Launcher.main(Launcher.java:109)

on mac os x mavericks