division by zero error for categories

I re-weighted the categories and made some 0. click save, and now i cannot access Gradebook:

Exception

  InvalidOperation: 0 / 0
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: <zope.browserpage.simpleviewclass.SimpleViewClass from /usr/lib/python2.7/dist-packages/schooltool/gradebook/browser/templates/gradebook_overview.pt object at 0xef2186c>
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/zope/browserpage/simpleviewclass.py", line 44, in __call__
  return self.index(*args, **kw)
File "/usr/lib/python2.7/dist-packages/zope/browserpage/viewpagetemplatefile.py", line 83, in __call__
  return self.im_func(im_self, *args, **kw)
File "/usr/lib/python2.7/dist-packages/zope/browserpage/viewpagetemplatefile.py", line 51, in __call__
  sourceAnnotations=getattr(debug_flags, 'sourceAnnotations', 0),
File "/usr/lib/python2.7/dist-packages/zope/pagetemplate/pagetemplate.py", line 113, in pt_render
  strictinsert=0, sourceAnnotations=sourceAnnotations)()
Template "/usr/lib/python2.7/dist-packages/schooltool/gradebook/browser/templates/gradebook_overview.pt"
File "/usr/lib/python2.7/dist-packages/zope/tal/talinterpreter.py", line 271, in __call__
  self.interpret(self.program)
File "/usr/lib/python2.7/dist-packages/zope/tal/talinterpreter.py", line 343, in interpret
  handlers[opcode](self, args)
File "/usr/lib/python2.7/dist-packages/zope/tal/talinterpreter.py", line 888, in do_useMacro
  self.interpret(macro)
File "/usr/lib/python2.7/dist-packages/zope/tal/talinterpreter.py", line 343, in interpret
  handlers[opcode](self, args)
File "/usr/lib/python2.7/dist-packages/zope/tal/talinterpreter.py", line 533, in do_optTag_tal
  self.do_optTag(stuff)
File "/usr/lib/python2.7/dist-packages/zope/tal/talinterpreter.py", line 518, in do_optTag
  return self.no_tag(start, program)
File "/usr/lib/python2.7/dist-packages/zope/tal/talinterpreter.py", line 513, in no_tag
  self.interpret(program)
File "/usr/lib/python2.7/dist-packages/zope/tal/talinterpreter.py", line 343, in interpret
  handlers[opcode](self, args)
File "/usr/lib/python2.7/dist-packages/zope/tal/talinterpreter.py", line 946, in do_defineSlot
  self.interpret(slot)
File "/usr/lib/python2.7/dist-packages/zope/tal/talinterpreter.py", line 343, in interpret
  handlers[opcode](self, args)
File "/usr/lib/python2.7/dist-packages/zope/tal/talinterpreter.py", line 533, in do_optTag_tal
  self.do_optTag(stuff)
File "/usr/lib/python2.7/dist-packages/zope/tal/talinterpreter.py", line 518, in do_optTag
  return self.no_tag(start, program)
File "/usr/lib/python2.7/dist-packages/zope/tal/talinterpreter.py", line 513, in no_tag
  self.interpret(program)
File "/usr/lib/python2.7/dist-packages/zope/tal/talinterpreter.py", line 343, in interpret
  handlers[opcode](self, args)
File "/usr/lib/python2.7/dist-packages/zope/tal/talinterpreter.py", line 819, in do_loop_tal
  iterator = self.engine.setRepeat(name, expr)
File "/usr/lib/python2.7/dist-packages/zope/tales/tales.py", line 682, in setRepeat
  expr = self.evaluate(expr)
File "/usr/lib/python2.7/dist-packages/zope/tales/tales.py", line 696, in evaluate
  return expression(self)
Template "/usr/lib/python2.7/dist-packages/schooltool/gradebook/browser/templates/gradebook_overview.pt", line 14, column 6
  Expression: <PathExpr standard:u'view/table'>
File "/usr/lib/python2.7/dist-packages/zope/tales/expressions.py", line 217, in __call__
  return self._eval(econtext)
File "/usr/lib/python2.7/dist-packages/zope/tales/expressions.py", line 211, in _eval
  return ob()
File "/usr/lib/python2.7/dist-packages/schooltool/gradebook/browser/gradebook.py", line 583, in table
  student)
File "/usr/lib/python2.7/dist-packages/schooltool/gradebook/gradebook.py", line 260, in getWorksheetTotalAverage
  adjusted_weights[key] /= total_percentage
File "/usr/lib/python2.7/decimal.py", line 1319, in __truediv__
  return context._raise_error(DivisionUndefined, '0 / 0')
File "/usr/lib/python2.7/decimal.py", line 3844, in _raise_error
  raise error(explanation)