This will hit you if a subscriber needs a table of content and you installed version 3.2.7 of the digests extension. What happened was that in cleaning up the code I decided that the assign_vars method should be simplified to use assign_var when only one template variable needs to be connected to a template. Except that the messenger class used for email is a subset of the template class and it doesn’t support the assign_var method. So it fails. You can see the patch here.