Same for me, Eclipse 3.5.0 with jdk 6 and Drools 5 (contained in JBoss Tools) under Windows XP. I get exactly the same exception when trying to debug. With Eclipse 3.4.2 it works (the other environment keeps the same). Cheers Bernd P.S: Maybe the wrong place and time, but a forum for drools users would be much more handy to use than the mailing list ;-) -- Hi, I am using Eclipse 3.5.0, jdk 1.6.0_13-b03 and Drools 5.0 all on Windows XP Professional SP3. Jakob On Sat, Aug 22, 2009 at 11:47 PM, Kris Verlaenen < kris.verlae...(a)cs.kuleuven.be&gt; wrote: