Haystack SearchQuerySet won't filter on a CharField with one character (Whoosh / django-haystack)

Haystack SearchQuerySet won't filter on a CharField with one character
(Whoosh / django-haystack)

I'm using Django 1.5.1 with django-haystack 2.1.0 and the whoosh 2.5.2
backend:
models.py:
GENDER_CHOICES = (
(u'M', u'Male'),
(u'F', u'Female'),
)
class Applicant(models.Model):
gender = models.CharField(max_length=1, choices=GENDER_CHOICES)
first_name = models.CharField(max_length=64)
last_name = models.CharField(max_length=64)
search_indexes.py:
class ApplicantIndex(indexes.SearchIndex, indexes.Indexable):
text = indexes.CharField(document=True,use_template=True)
gender = indexes.CharField(model_attr="gender")
search template
{{ object.first_name }}
{{ object.last_name }}
In the django shell i'm trying following:
>>> from haystack.query import SearchQuerySet
>>> sqs=SearchQuerySet()
>>> sqs
[<SearchResult: tooldb.applicant (pk=u'1')>, <SearchResult:
tooldb.applicant (pk=u'2')>]
>>> sqs[0].gender
u'M' #<-- So this seems to be indexed
#but when i try:
>>> sqs.filter(gender='M')
[] #<-- I get nothing ... ?
I tried it with other CharFields without choices and max_lenght > 1, no
problem at all, haystack filters like it should.
What am I missing?