Rules: Doesn't check for empty token

Trying to use Rules to add energy to nodes that have none doesn't work. After hours of trying to figure out how to use rules with radioactivity, I finally found that the algorithm field requires a token replacement (unless you want to overwrite existing energy) and that the token must not evaluate to null. I think the offending lines of code begin around line 64 in radioactivity_rules.module:

  // FIXME: messy regexp
  // This will remove everything else other than 0-9 + - * / and functions sqrt, sin, cos, pow and tan
  preg_match_all("/([0-9\+\-\/\*\(\)\.\ ]|sqrt *\(|sin *\(|cos *\(|pow *\(|tan *\()/i", $algorithm, $matches);
  $clean = implode('', $matches[1]);
  
  ob_start();
  print eval('echo ' . $clean . ';');

If the token is [story_node:radioactivity-energy-1], the algorithm can be '[story_node:radioactivity-energy-1] + 10'. If the node has 5 in energy, 'echo $clean' would evaluate '5 + 10' and everything would be ok. But if the node has never received any energy, print eval would run echo on ' + 10', which is not a double and you're left with nothing, including no error messages.