How to use the pysle.pronunciationtools.NullPhoneError function in pysle

# Unify rhotics
        if 'r' in phone:
            phone = 'r'

        phone = phone.lower()

        # Unify vowels
        if isletool.isVowel(phone):
            phone = u'V'

        # Only represent the string by its first letter
        try:
            phone = phone[0]
        except IndexError:
            raise NullPhoneError()

        # Unify vowels (reducing the vowel to one char)
        if isletool.isVowel(phone):
            phone = u'V'

        retList.append(phone)

    return retList