Error when changing M2_HOME on linux (at least :) )

When I tried to change the value of M2_HOME directory, I get the following error

-bash-3.2$ mvn -v
Exception in thread "main" java.lang.NoClassDefFoundError: org/codehaus/plexus/classworlds/launcher/Launcher
Caused by: java.lang.ClassNotFoundException: org.codehaus.plexus.classworlds.launcher.Launcher
at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:276)
at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)

My m2_home includes the same settings.xml
If I unset the m2_home, it's find :

-bash-3.2$ mvn -v
Apache Maven 3.0-alpha-7 (r921173; 2010-03-09 23:31:07+0100)
Java version: 1.6.0_07
Java home: /opt/sun/jdk1.6.0_07/jre
Default locale: en_US, platform encoding: UTF-8
OS name: "linux" version: "2.6.18-164.el5" arch: "amd64" Family: "unix"