paratest: Using @group throws exception

Whenever we use the @group annotation we encounter the following exception

[RuntimeException]                                                                                                                      
  Log file /tmp/PT_YQNOiy is empty.                                                                                                       
                  This means a PHPUnit process was unable to run /src/test/Components/CompetitionFilter/FilterTest.php  
                  Maybe there is more than one class in this file.

Having traced the code in ParaTest we’ve found where the exception is being thrown and that the Reader->hasResults() method is returning false, thus causing the exception.

However, should this code even be being executed as the test in the Exception does not contain the requested @group annotation?

With the following lines commented out - the tests work as expected, but there could be underlying issues to which we’re not aware.

       if (!$reader->hasResults()) {
            throw new \RuntimeException("Log file " . $test->getTempFile() . " is empty.
                This means a PHPUnit process was unable to run " . $test->getPath() . "
                Maybe there is more than one class in this file.");
        }
        $this->results->addReader($reader);

Any advice would be appreciated.