plone/plone.app.textfield — pull request #8 by ampsport (2014-03-10 23:59:31+00:00)
Title: Do not give an error when the raw value is not unicode and isn't ascii. ...

...In that case, encode as unicode then decode as the proper string, bang head on desk.

I am not sure how the mechanics here are working. There is likely a better fix which stops data from being stored in a bad format to begin with, but I am honestly not familiar enough with these mechanics to understand what is going on.