NPE when building documentation

I'm getting NPE when building site docs, with no indication in the stack-trace as to what has gone wrong. Different things happen if I build from clean or build again on top of a failed build.

In the site directory, I type: mvn site
This builds some archives for child modules and then starts chewing on the dox files. It fails part way with a NPE. You can get the code that triggers this from subversion:

http://metagenome.ncl.ac.uk/subversion/instantsoap/trunk/instantsoap/site/
revision 598

The stack-trace looks like this:

[INFO] [site:site]
[INFO] ------------------------------------------------------------------------
[ERROR] FATAL ERROR
[INFO] ------------------------------------------------------------------------
[INFO] null
[INFO] ------------------------------------------------------------------------
[INFO] Trace
java.lang.NullPointerException
at org.apache.maven.doxia.module.xdoc.XdocParser.handleEndTag(XdocParser.java:467)
at org.apache.maven.doxia.parser.AbstractXmlParser.parseXml(AbstractXmlParser.java:92)
at org.apache.maven.doxia.parser.AbstractXmlParser.parse(AbstractXmlParser.java:53)
at org.apache.maven.doxia.module.xdoc.XdocParser.parse(XdocParser.java:99)
at org.apache.maven.doxia.DefaultDoxia.parse(DefaultDoxia.java:59)
at org.apache.maven.doxia.siterenderer.DefaultSiteRenderer.renderDocument(DefaultSiteRenderer.java:342)
at org.apache.maven.doxia.siterenderer.DoxiaDocumentRenderer.renderDocument(DoxiaDocumentRenderer.java:46)
at org.apache.maven.doxia.siterenderer.DefaultSiteRenderer.renderModule(DefaultSiteRenderer.java:269)
at org.apache.maven.doxia.siterenderer.DefaultSiteRenderer.render(DefaultSiteRenderer.java:101)
at org.apache.maven.plugins.site.SiteMojo.renderLocale(SiteMojo.java:129)
at org.apache.maven.plugins.site.SiteMojo.execute(SiteMojo.java:96)
at org.apache.maven.plugin.DefaultPluginManager.executeMojo(DefaultPluginManager.java:451)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoals(DefaultLifecycleExecutor.java:558)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalWithLifecycle(DefaultLifecycleExecutor.java:499)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoal(DefaultLifecycleExecutor.java:478)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalAndHandleFailures(DefaultLifecycleExecutor.java:330)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeTaskSegments(DefaultLifecycleExecutor.java:291)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.execute(DefaultLifecycleExecutor.java:142)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:336)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:129)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:287)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.codehaus.classworlds.Launcher.launchEnhanced(Launcher.java:315)
at org.codehaus.classworlds.Launcher.launch(Launcher.java:255)
at org.codehaus.classworlds.Launcher.mainWithExitCode(Launcher.java:430)
at org.codehaus.classworlds.Launcher.main(Launcher.java:375)