Create a custom field in wtForms

In my form I am trying to create a custom array field with choices.

The custom form field:

class CustomField(Field):
    widget = TextInput()
    def _value(self):
        if self.data:
            return u', '.join(self.data)
        else:
            return u''

    def process_formdata(self, valuelist):
        if valuelist:
            self.data = [x.strip() for x in valuelist[0].split(',')]
        else:
            self.data = []

The actual form calls the custom form field

class PostForm(Form):

    status = CustomField()

Whenever, I post data to PostForm it calls the custom field but does not pass in an any valuelist to process_formatdata. The custom field always returns a empty list.
Am I missing any thing here.