java.lang.ClassFormatError under IBM JVM

When run the jruby under IBM java 6, java.lang.ClassFormatError pop up. followings are the details

to trigger the problem, just run a rake session, even without a rakefile

>jruby -S rake -T
ClassLoader.java:265:in `defineClass': java.lang.ClassFormatError:
JVMCFRE068 class name is invalid; class=rubyjit
$initialize_9FFD7936C93B2FA4E78CC51FF56BE551BF6DD8A6#block_0$RUBY
$_file_, offset=0
from JRubyClassLoader.java:76:in `defineClass'
from InvocationMethodFactory.java:1351:in `endClassWithBytes'
from InvocationMethodFactory.java:1335:in `endCallWithBytes'
from InvocationMethodFactory.java:832:in `getBlockCallback'
from RuntimeHelpers.java:165:in `createBlockCallback'
from RuntimeHelpers.java:262:in `createCompiledBlockBody'
from RuntimeHelpers.java:252:in `createCompiledBlockBody'
from RuntimeCache.java:421:in `createBlockBody'
from RuntimeCache.java:66:in `getBlockBody'
from AbstractScript.java:93:in `getBlockBody0'
from D:/jruby/lib/ruby/site_ruby/1.8/rubygems/requirement.rb:97:in
`_file_'
from D:/jruby/lib/ruby/site_ruby/1.8/rubygems/requirement.rb:-1:in
`_file_'
...

jruby -v
jruby 1.6.0 (ruby 1.8.7 patchlevel 330) (2011-03-15 f3b6154) (IBM J9
VM 1.6.0) [Windows XP-x86-java]
java version
>%JAVA_HOME%\bin\java -version

java version "1.6.0"
Java(TM) SE Runtime Environment (build pwi3260sr1-20080416_01(SR1))
IBM J9 VM (build 2.4, J2RE 1.6.0 IBM J9 2.4 Windows XP x86-32
jvmwi3260-20080415_18762 (JIT enabled, AOT enabled)
J9VM - 20080415_018762_lHdSMr
JIT - r9_20080415_1520
GC - 20080415_AA)
JCL - 20080412_01