'Invoice object' has no attribute "invoiceLign_set

I have the following 3 models.
The “amount_to_plan” method for Business works fine while the “invoice_amountVATe” does not.
I do not understand because it seems to me they are really equivalent (the first “binding” Business and “Invoice” and the latter binding “Invoice” with “InvoiceLign”

What could be wrong?

class Business(models.Model):
    name = models.CharField('Affaire',max_length=200, null = True, blank=True)
    customer = models.ForeignKey(Customer,on_delete=models.CASCADE,verbose_name='Client',
        null = True,blank = True)
       amount = models.DecimalField('Montant',max_digits=9, decimal_places=2, null=True,blank=True)
    def amount_to_plan(self):
        invoice = self.invoice_set.all()
        a = self.amount
        for i in invoice:
            a -= i.amount
        return a   
    def get_absolute_url(self):
        return reverse('customer_detail_form', kwarg={'pk':self.pk})
    class Meta:
        ordering = ['ponderation','amount']
        verbose_name = 'Affaire'
class Invoice(models.Model):
    number = models.CharField('Numéro de facture', max_length=10, null=True, blank=True)
    issuance_date = models.DateField('Date de facture', default=datetime.date.today)
    amount = models.DecimalField('Montant',max_digits=11, decimal_places=2, default=0)
    business = models.ForeignKey(Business, on_delete=models.CASCADE, verbose_name='Commande')
    def invoice_amountVATe(self):
        invoice_ligns = self.invoiceLign_set.all()
        invoice_amountVATe = 0
        for i in invoice_ligns:
            invoice_amountVATe += i.amountVATe()
        return invoice_amountVATe
    class Meta:
        ordering = ['issuance_date']
        verbose_name = 'Facture'
    def __str__(self):
        return self.label
    def get_absolute_url(self):
        return reverse('invoice_detail', args=[str(self.id)])    

class InvoiceLign(models.Model):
    invoice = models.ForeignKey(Invoice, on_delete= models.CASCADE)
    label = models.CharField('Désignation', max_length=200)
    unit_price = models.DecimalField('Prix unitaire',max_digits=9, decimal_places=2)
    quantity = models.IntegerField('Quantité')
    VAT_rate = models.DecimalField('Taux de TVA', max_digits=5, decimal_places=2, default=20)
    def amountVATe(self):
        # Montant HT de la ligne
        return self.quantity*self.unit_price
    def VAT_amount(self):
        # Montant de TVA de la ligne
        return self.amountVATe*self.VAT_rate/100
    def amountVATi(self):
        # Montant TTC de la ligne
        return self.amountVATe + self.VAT_amount
    class Meta:
        ordering = ['label']
        verbose_name = 'Ligne de facture'
    def __str__(self):
        return self.label    

The reverse query manager uses a lowercase version of the model name. It would be invoicelign_set.