Django: MultipleChoiceField in admin to carry over previously saved values

I am having troubles to carry over previously selected items in a ModelForm in the admin. I want to use the forms.CheckboxSelectMultiple widget since that is the most straightforward UI in this usecase. It works as far that when saving, the values are stored. But when editing the previously saved item, the values previously saved in this field are not reflected in the widget.

UI Example:

After posting (editing that item, returns it blank):

However, when not using the widget but a regular CharField when editing the item it looks like:

So for some reason the values are not represented by the checkbox widget?

Here's my simplified setup, models.py

POST_TYPES = (
    ('blog',      'Blog'),
    ('portfolio',     'Portfolio'),
    ('beeldbank',     'Beeldbank'),
)

class Module(models.Model):
    title         =   models.CharField(max_length=100, verbose_name='title')        
    entriesFrom     =   models.CharField(max_length=100)

    def __unicode__(self):
       return self.title

forms.py:

class ModuleForm(forms.ModelForm):
    entriesFrom = forms.MultipleChoiceField(
            choices=POST_TYPES, 
            widget=CheckboxSelectMultiple, 
            label="Pull content from", 
            required=False,
            show_hidden_initial=True) 

    class Meta:
        model = Module

    def __init__(self, *args, **kwargs):
        super(ModuleForm, self).__init__(*args, **kwargs)

        if kwargs.has_key('instance'):
            instance = kwargs['instance']
            self.fields['entriesFrom'].initial = instance.entriesFrom

            logging.debug(instance.entriesFrom)

admin.py

class ModuleAdmin(admin.ModelAdmin):
    form = ModuleForm

So when editing a previously saved item with say 'blog' selected, debugging on init returns me the correct values on self.fields['entriesFrom'] ([u'blog',]), but its not reflected in the checkboxes (nothing is shown as selected) in the admin.

edit

updated the ModuleForm class to pass on initial values, but nothing still gets pre-populated whilst there are a few values in the initial value ("[u'blog']").

Solution:

Setting the choices by a integer, instead of a string.

POST_TYPES = (
    (1,     'Blog'),
    (2,     'Portfolio'),
    (3,     'Beeldbank'),
)

Damn, that wasn't worth breaking my skull over.