Attribute error after running "doc._.has_grammar_error"

[SRIKARHI]

AttributeError: [E046] Can't retrieve unregistered extension attribute 'has_grammar_error'. Did you forget to call the set_extension method?

I have cloned this repo and ran this script:
import spacy
from spacy_grammar.grammar import Grammar

nlp = spacy.load('en')
grammar = Grammar(nlp)
nlp.add_pipe(grammar)
doc = nlp('I can haz cheeseburger.')
doc._.has_grammar_error

I am getting this error.Kindly help

[buttchurch]

I'm getting this error as well! Seconded