ValueError exception raised when CC number contains non numeric characters

This is the error I'm getting.
```
File "/projects/.python-env/projectfolder/src/django-merchant-custom/billing/utils/credit_card.py", line 35, in is_luhn_valid
   num = [int(x) for x in str(self.number)]

ValueError: invalid literal for int() with base 10: '`'
```