I can see how to add an error message to a field when using forms, but what about model form?

This is my test model:

class Author(models.Model):
    first_name = models.CharField(max_length=125)
    last_name = models.CharField(max_length=125)
    created = models.DateTimeField(auto_now_add=True)
    updated = models.DateTimeField(auto_now=True)

My model form:

class AuthorForm(forms.ModelForm):
    class Meta:
        model = Author

The error message on the fields: first_name and last_name is:

This field is required

How do I change that in a model form?

django django-forms
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