Fatal error: Only variables can be passed by reference in /var/www/beta/modules/workflow/workflow.module on line 129

Drupal version: 4.7.0 Beta 5
MySQL version: 4.0.24
Platform: Ubuntu Breezy
PHP version: 5.0.5

Having associated a particular workflow with Book nodes, I get the following error whenever I try to create a book:

Fatal error: Only variables can be passed by reference in /var/www/beta/modules/workflow/workflow.module on line 129

I gather this is a common problem with other applications under PHP 5.

Code in question:

      // get new state
      $sid = $node->workflow;
      if (!$sid && $op == 'insert') { // if not specified, use first valid
        $sid = array_shift(array_keys(workflow_field_choices($node))); //THIS IS LINE 129
      }