How do you add a non-editable field to a custom admin form in Django

I am trying to add an editable=False field to a custom admin form, but I am getting an error:

django.core.exceptions.FieldError: 'help_num' cannot be specified for 
Investigation model form as it is a non-editable field

This is true, in my model I have it set as such:

models.py

help_num = models.CharField(max_length=17, unique=True, default=increment_helpdesk_number, editable=False)

forms.py

class HelpDeskModelForm(forms.ModelForm):

    class Meta:
      model = HelpDesk
      fields = [
          "help_num",
          "help_types",
           ...
          "help_summary"
          ]

admin.py

class HelpDeskModelAdmin(admin.ModelAdmin):
    readonly_fields=('help_num',)
    form = HelpDeskModelForm

I added the readonly to admin.py, but am still getting the error. What am I doing wrong?