Cannot instantiate abstract class RulesException

I'm using wsclient 7.x-1.0-alpha4 and rules 7.x-2.0-rc1. When I attempt to trigger a rule that calls a REST web service, PHP returns this error:

Fatal error: Cannot instantiate abstract class RulesException in /drupal_root/sites/all/modules/wsclient/wsclient.rules.inc on line 176

Commenting out line 176 allows the rule to fire properly, but I'm not enough of a PHP guru to fix it properly. Looking around, it seems like there was a recent commit to rules related to how rules supports translations of exception messages. See http://drupal.org/node/1217504. Not sure if that's the root cause or not, but I hope it helps.