title: Django error - __str__ returned non-string (type User)
body: <p>I have the below 2 models - User and PhoneProfile.</p>
  <p>I can successfully create records for User. When i browse to PhoneProfile using admin console, i can see the foreign key present. On selection of an entry and completion of the rest of the fields, i get the error below.</p>
  <pre><code>class User(AbstractBaseUser):
      phone_regex     = RegexValidator(regex = r'^\+?1?\d{2,14}$', message = &quot;Phone number must be in the format: '+xxxxxxxxxx'.&quot;)
      phone           = models.CharField(validators = [phone_regex], max_length = 15, unique = True)
      first_name      = models.CharField(max_length = 50, blank = False, null = False)
      last_name       = models.CharField(max_length = 50, blank = False, null = False)
      email           = models.CharField(max_length = 100, blank = True, null = True)
      first_login     = models.BooleanField(default = False)
      active          = models.BooleanField(default = True)
      staff           = models.BooleanField(default = True)
      admin           = models.BooleanField(default = False)
      timestamp       = models.DateTimeField(auto_now_add=True)
  
      USERNAME_FIELD  = 'phone'
      REQUIRED_FIELDS = []
  
      objects         = UserManager()
  </code></pre>
  <pre><code>class PhoneProfile(models.Model):
      registered_phone =  models.ForeignKey(User, on_delete=models.CASCADE, related_name='registered_phone')
      notify_enable_low_bal = models.BooleanField()
      notify_amount_low_bal = models.DecimalField(decimal_places=2, max_digits=10000, default='50', null=True)
  </code></pre>
  <p>Error message when i attempt to create an entry in PhoneProfile model</p>
  <pre><code>__str__ returned non-string (type User)
  Request Method: POST
  Request URL:    http://localhost:8000/admin/smartrecharge/phoneprofile/add/
  Django Version: 3.0.7
  Exception Type: TypeError
  Exception Value:    
  __str__ returned non-string (type User)
  Exception Location: /Users/django-backend/venv/lib/python3.7/site-packages/django/contrib/admin/options.py in log_addition, line 807
  Python Executable:  /Users/django-backend/venv/bin/python
  Python Version: 3.7.7
  Python Path:    
  
  </code></pre>
  <p>Any guide will be appreciated. Thanks</p>