How to debug faulty phpunit HHVM?

I am trying to run our testsuite in HHVM. Unfortunately, this won't work. PHPunit starts and then just exits without running one test (exit code 255). The error message is missing and I don't know how to figure out what is wrong:

$> hhvm phpunit.phar 
PHPUnit 4.4.1 by Sebastian Bergmann.

Configuration read from /home/splitbrain/public_html/dw-2014-01-07/_test/phpunit.xml

$> echo $?
255

      

        
        
        
      

    

I've tried running single test files, but it doesn't seem to matter which one I use. I am guessing that something went wrong in our bootstrap, but I don't know how to figure out what.

Is there a way to get HHVM to log a traceback of a function call or something to figure out what happened last before it exited?