UnicodeError:"label empty or too long"

Adding a new person:
     <School><People (under my school year)><Teacher (under the Add, without clicking on 'Done')>

The error shows up, I realize that I should have clicked on 'Done' first, but then it still should not give the error list that follows:
-----------------
Server Error
An internal server error has occurred. This most likely means that you have found a bug in SchoolTool. Please report it on the issue tracker.

Exception
UnicodeError: label empty or too long

Traceback
File "/usr/lib/python2.7/dist-packages/zope/publisher/publish.py", line 132, in publish
  result = publication.callObject(request, obj)
File "/usr/lib/python2.7/dist-packages/zope/app/publication/zopepublication.py", line 207, in callObject
  return mapply(ob, request.getPositionalArguments(), request)
File "/usr/lib/python2.7/dist-packages/zope/publisher/publish.py", line 107, in mapply
  return debug_call(obj, args)
Extra information: <schooltool.skin.flourish.zcml_content.FlourishPersonAddView_addTeacher.html object at 0x7fb2e8a20590>
File "/usr/lib/python2.7/dist-packages/zope/publisher/publish.py", line 113, in debug_call
  return obj(*args)
File "/usr/lib/python2.7/dist-packages/z3c/form/form.py", line 215, in __call__
  self.update()
File "/usr/lib/python2.7/dist-packages/schooltool/basicperson/browser/person.py", line 929, in update
  self.buildFieldsetGroups()
File "/usr/lib/python2.7/dist-packages/schooltool/basicperson/browser/person.py", line 952, in buildFieldsetGroups
  _('Demographics'), list(self.getDemoFields())),
File "/usr/lib/python2.7/dist-packages/schooltool/basicperson/browser/person.py", line 865, in getDemoFields
  fields += field_desc.makeField()
File "/usr/lib/python2.7/dist-packages/schooltool/basicperson/demographics.py", line 216, in makeField
  vocabulary=IDNAVocabulary.fromValues(self.items)
File "/usr/lib/python2.7/dist-packages/zope/schema/vocabulary.py", line 99, in fromValues
  terms = [cls.createTerm(value) for value in values]
File "/usr/lib/python2.7/dist-packages/schooltool/basicperson/demographics.py", line 202, in createTerm
  token = value.encode('idna')
File "/usr/lib/python2.7/encodings/idna.py", line 164, in encode
  result.append(ToASCII(label))
File "/usr/lib/python2.7/encodings/idna.py", line 73, in ToASCII
  raise UnicodeError("label empty or too long")