Dexterity Choice Field (first element be considered a non-value)

Are you referring to this?

If so, based on it, this is what I did:
Added the following in file called policy.py in my.product/src/my/product/policy.py

"""Module where all interfaces, events and exceptions live."""

from ncdhhscontenttype.ncssinfo import _
from zope import schema
from zope.interface import Interface
from zope.publisher.interfaces.browser import IDefaultBrowserLayer
from plone.supermodel import model
from Acquisition import aq_inner
from plone import api
from Products.Five import BrowserView
from z3c.form import validator
from zope.interface import Invalid


class IMyProductLayer(IDefaultBrowserLayer):
    """Marker interface that defines a browser layer."""

class CustomValidator(validator.SimpleFieldValidator):
    def validate(self, value):
        super(CustomValidator, self).validate(value)
        if value:
            if ['Please choose one:']:
                raise Invalid(u'Value is not valid')

In my schema model file, policy.xml file, I called it using the following:

<field name="choices" type="zope.schema.Choice" indexer:searchable="true">
    <description/>
    <required>True</required>
    <title>Choices</title>
    <values>
        <element>Please choose one:</element>
        <element>Choice A</element>
        <element>Choice B</element>
        <element>Choice C</element>
    </values>
    <form:validator="my.product.policy.CustomValidator">
</field>

Error message:

SupermodelParseError: error parsing attribute name

It is referring to the line that has the form:validator statement in my policy.xml file. I am sure I am missing something. Any help would be most appreciated. Thank you.