The reason of this bug is:
for multiple choice pools the form uses MultipleChoiceField and CheckboxSelectMultiple widget, but for single choice pools it uses ChoiceField and RadioSelect. RadioSelect+ChoiceField returns only the choice itself, not a list (string id in this case).
When we ask len("13"), for example, it returns 2, so validation fails.
The best solution I've found so far is a variation of sedah's solution (see pull request #13), but with a reasonable check (not the "instance of" magic):
I also tried (a dumb idea) to use MultipleChoiceField with RadioSelect widget but them doesn't work together :)