Serializer is_valid() keeps saying that primary key related field is required even when it is provided???

[123epsilon]

My issue is exactly the same one as described here: https://django.fun/en/qa/382239/

I have no idea why this is happening, I have a model and serializer:

class OpeningHours(models.Model):

    businessProfile = models.ForeignKey('BusinessProfile', on_delete=models.CASCADE)

    weekday = models.IntegerField(choices=WEEKDAYS)
    from_hour = models.TimeField()
    to_hour = models.TimeField()

    class Meta:
        ordering = ('weekday', 'from_hour')
        unique_together = ('businessProfile', 'weekday', 'from_hour', 'to_hour')
        app_label = 'shft'

    def __unicode__(self):
        return u'%s: %s - %s' % (self.get_weekday_display(),
                                 self.from_hour, self.to_hour)

class OpeningHoursSerializer(serializers.ModelSerializer):
    businessProfile = serializers.PrimaryKeyRelatedField(read_only=True
    
    class Meta:
        model = OpeningHours
        fields = '__all__'
        validators = [
            serializers.UniqueTogetherValidator(
                queryset=model.objects.all(),
                fields=('businessProfile', 'weekday', 'from_hour', 'to_hour'),
                message=("Duplicate Opening Hours not allowed")
            )
        ]

When I try to create an object, I can't get past the serializer validation stage. If I create a BusinessProfile object with id 1, then I should be able to create an opening hours entry like this:

url = reverse('create-opening-hours')

data = {
	'businessProfile': self.bp1_pk, #value recorded when creating the business profile in testing
	'weekday': 1,
	'from_hour': '12:05',
	'to_hour': '5:00'
}

# create opening hours
self.client.credentials(HTTP_AUTHORIZATION=f'Bearer {self.acc1}')
response = self.client.post(url, data, format='json')

Instead I get

<Response status_code=400, "application/json"> {'businessProfile': [ErrorDetail(string='This field is required.', code='required')]}

What am I doing wrong here? A similar strategy works without error when I am using a OneToOneField but in this case I need the related field to be a ForeignKey.