I've had luck with adding this as a sibling to the RTE dialog's
Note that if text has already been authored with
and tags you will need to go in and add a space somewhere in the
rte editor to pick up the change and swap the tags.
I had the same exception crop up mysteriously too. It bugged our dev
server but locally everything was fine. The problem was resolved by
hitting the Refresh Packages button in /system/console/configMgr