WTForms "too many values to unpack" with SelectField

I'm using WTForms and I'm trying to display a SelectField, but I get the following error:

>>> form.status()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python26\Lib\site-packages\wtforms\fields.py", line 136, in __call__
    return self.widget(self, **kwargs)
  File "C:\Python26\Lib\site-packages\wtforms\widgets.py", line 237, in __call__
    for val, label, selected in field.iter_choices():
  File "C:\Python26\Lib\site-packages\wtforms\fields.py", line 390, in iter_choices
    for value, label in self.choices:
ValueError: too many values to unpack

Here's my form:

class TestForm(Form):
    status = SelectField(u'Status', choices=Test.statuses())

The Test.statuses static method returns a list of strings. What am I doing wrong?