Problem generating the docs

Hi, got maven to work over proxy, but still facing a strange problem:

$ mvn docbook:transform

[INFO] Scanning for projects…

[INFO] Searching repository for plugin with prefix: ‘docbook’.

[INFO] ------------------------------------------------------------------------

[INFO] Building DHIS2 Documentation system

[INFO] task-segment: [docbook:transform]

[INFO] ------------------------------------------------------------------------

[INFO] [docbook:transform]

[INFO] ------------------------------------------------------------------------

[ERROR] FATAL ERROR

[INFO] ------------------------------------------------------------------------

[INFO] null

[INFO] ------------------------------------------------------------------------

[INFO] Trace

java.lang.NullPointerException

at java.util.Hashtable.put(Hashtable.java:394)

at java.util.Properties.setProperty(Properties.java:143)

at java.lang.System.setProperty(System.java:731)

at org.codehaus.mojo.docbook.TransformMojo.execute(TransformMojo.java:130)

at org.apache.maven.plugin.DefaultPluginManager.executeMojo(DefaultPluginManager.java:483)

at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoals(DefaultLifecycleExecutor.java:678)

at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeStandaloneGoal(DefaultLifecycleExecutor.java:553)

at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoal(DefaultLifecycleExecutor.java:523)