While working with django I recently came across the following error:
Caught an exception while rendering: coercing to Unicode: need string or buffer, NoneType found
Google doesn’t turn up much.  What is did turn up was a lot of “you’re missing the __unicode__() method on your model” responses.  All my models had the method so that wasn’t it.  I eventually tracked the problem down to the __unicode__ method for one of the models returning a field that may contain a NULL (or blank) value.  Updating the method to return a field that was never null fixed the problem.