Что вызывает эту ошибку в моем коде 'unsupported operand type(s) for +=: 'NoneType' and 'int''.

Я пытаюсь создать инвестиции, а также обновить баланс инвестиций, но получаю эту ошибку NoneType. Ниже приведен код моей модели и представления.

class Investment(models.Model):
    user = models.ForeignKey(
        User, on_delete=models.CASCADE, null=True, blank=True)
    amount = models.IntegerField(null=True)
    balance = models.IntegerField(null=True, blank=True)
    investment_id = models.CharField(max_length=10, null=True, blank=True)
    plan = models.ForeignKey(Plan, on_delete=models.CASCADE, null=True)
    active = models.BooleanField(default=True)
    created_at = models.DateTimeField(auto_now=True, null=True)

    def __str__(self):
        return str(self.investment_id)



def create_investment_view(request):
    if request.method == 'POST':
        investment_form = InvestmentForm(request.POST)
        if investment_form.is_valid():
            investment = investment_form.save(commit=False)
            investment.balance += investment.amount
            investment.balance.save()
            investment.save()
            messages.success(request, 'your investment is successfull')
        else:
            messages.success(request, 'your investment is not successfull! Try again.')
    else:
        investment_form = InvestmentForm()
    context = {'investment_form': investment_form}
    return render(request, 'create-investment.html', context)

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