[maven integration] Using with latest maven?

Hello,

I am trying to use the maven plugin of cactus. I use the 4.0 model version  
and the latest maven version.
I added this to my projects pom.xml (I use version 1.7 because 1.7.1 is  
not supported by my maven version (wrong pom version)):
   <build>
     <plugins>
       <plugin>
         <groupId>cactus</groupId>
         <artifactId>cactus-maven</artifactId>
         <version>1.7</version>
       </plugin>
     </plugins>
   </build>
and tried to execute "mvn cactus:test".
This is what I get:
[INFO] Scanning for projects...
[INFO] Searching repository for plugin with prefix: 'maven'.
[INFO]  
------------------------------------------------------------------------
[ERROR] FATAL ERROR
[INFO]  
------------------------------------------------------------------------
[INFO] null
[INFO]  
------------------------------------------------------------------------
[INFO] Trace
java.lang.NullPointerException
         at  
org.apache.maven.plugin.DefaultPluginManager.addPlugin(DefaultPluginM
anager.java:292)
         at  
org.apache.maven.plugin.DefaultPluginManager.verifyVersionedPlugin(De
faultPluginManager.java:198)
         at  
org.apache.maven.plugin.DefaultPluginManager.verifyPlugin(DefaultPlug
inManager.java:163)

I dont know if this is an issue with the maven plugin or with maven  
itself. I will also post this mail to the Maven mailing list in a fews  
days.
Well. Anyone got that running?
Dont know what to try next, and I really would like to use maven with  
cactus integration.

thanks :)
Dennis Kempin