ValueError: invalid literal for int() with base 10:

Hello, I came across this error:

"ValueError: invalid literal for int() with base 10:" after I made a field in my models.py a ForeignKey. Below is my models.py

from django.conf import settings

from django.db import models

from django.core.urlresolvers import reverse

from django.utils import timezone

import datetime

from contacts.models import Contact

#from django.contrib.auth.models import User

# Create your models here.

class Invoice(models.Model):

    created_at = models.DateTimeField(auto_now_add=True)

    their_company = models.ForeignKey(Contact, on_delete=models.CASCADE,blank=True,

    null=True,)

    invoice_number = models.CharField(max_length=50, default='')

    bill_to = models.CharField(max_length=255, default='')

    created_by = models.ForeignKey(settings.AUTH_USER_MODEL)

    

    def save(self, *args, **kwargs):

        if not self.id:

            today = datetime.date.today()

            date_str = datetime.datetime.strftime(today, '%y%m%d')

            doc_str = 'ACIN'

            last_invoice = Invoice.objects.filter(invoice_number__startswith=doc_str).order_by('invoice_number').last()

            if last_invoice:

                last_invoice_num = last_invoice.invoice_number[-2:]

                new_invoice_num = int(last_invoice_num) + 1

                new_invoice_num = "%02d" % new_invoice_num

            else:

                new_invoice_num = '01'

            self.invoice_number = doc_str + date_str + new_invoice_num

        super(Invoice, self).save(*args, **kwargs)

        

        

    class Meta:

        ordering = ['invoice_number', ]

        

    def __str__(self):

        return self.invoice_number

        

    def get_absolute_url(self):

        return reverse("accounting:invoices:detail", kwargs={"pk": self.pk})

The 'their_company' field I think is whats giving the error. I think it is because it is trying to convert that field to an int but its not something that can be converted to an int. I'm new to django and I know I have something configured wrong somewhere but not sure where. 

Thanks for your help

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