crispy form no error with bootstrap beta

forms.py

from django import forms
from localflavor.gb.forms import GBPostcodeField

from crispy_forms.helper import FormHelper
from crispy_forms.layout import Submit, Layout, Div, Field, HTML, MultiWidgetField, Fieldset

class PostcodeForm(forms.Form):
    postcode = GBPostcodeField(label='Please enter your postcode',
                               error_messages={'invalid': 'Please enter a valid postcode'})

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.helper = FormHelper(self)
        self.helper.label_class = 'sr-only'
        self.helper.form_tag = False
        self.helper.layout = Layout(
            PrependedText('postcode', '<i class="fa fa-home"></i>', placeholder="Postcode", autofocus=""),
        )