Django queryset return true if the object was created on the last month

So I'm trying to make this query:

queryset = (
            Offer.objects.annotate(
                is_new=Case(
                    When(
                        created_at__month=[
                            now.month
                        ],
                        then=Value("True"),
                    ),
                    default=Value("False"),
                )
            )
            .values("is_new")
        )

Basically what i want is to return a new booleand field that tells me "true" if the offer was created on the last month. But I'm getting the following error:

TypeError: Field 'None' expected a number but got [1].

Does anyone know why this happens? or how can I fix it? Also I thank you If you have a better approach.

Thanks in advance :)

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