Hi everybody, 
I am using dynamic form fields and have figured out that they are behind an error that I get: 

`Request Method:    GET`

`Request URL:    https://blah.herokuapp.com/p/h8lpklyu/the_button/Survey/17/`

`Django Version:    2.2.12`

`Exception Type:    ImproperlyConfigured`

`Exception Value:    
Calling modelform_factory without defining 'fields' or 'exclude' explicitly is prohibited.
`

`Exception Location:    /app/.heroku/python/lib/python3.8/site-packages/django/forms/models.py in modelform_factory, line 545`
`Python Executable:    /app/.heroku/python/bin/python`
`Python Version:    3.8.10`

`Python Path:    
['/app/.heroku/python/bin',
 '/app',
 '/app/.heroku/python/lib/python38.zip',
 '/app/.heroku/python/lib/python3.8',
 '/app/.heroku/python/lib/python3.8/lib-dynload',
 '/app/.heroku/python/lib/python3.8/site-packages']`


My Survey page in pages.py looks as follows:


    class Survey(Page):
          form_model = 'player'
          form_fields = [ ]

    def is_displayed(self):
        player = self.player
        return player.treatment == "treatment1" or player.treatment == "treatment2"

    def get_form_fields(self):
        if self.player.treatment == "treatment2":
            if self.player.store_time != 0 and self.participant.vars["payoff1_self"]  == 1:
                return ['q0', 'q1','q_change']
            elif self.player.store_time != 0 and self.participant.vars["payoff1_self"]  ==10:
                return ['q0','q1','q_nochange']
            elif self.player.store_time == 0 and self.participant.vars["payoff1_self"]  ==10:
                return ['q0','q2','q_change']
            elif self.player.store_time == 0 and self.participant.vars["payoff1_self"] == 1:
                return ['q0','q2','q_nochange']
        elif self.player.treatment == "treatment1":
            if self.player.store_time != 0 and self.participant.vars["payoff1_self"]  == 1:
               return ['q0', 'q1',  'q_nochange']
            elif self.player.store_time != 0 and self.participant.vars["payoff1_self"]  ==10:
               return ['q0', 'q1', 'q_change']
            elif self.player.store_time == 0 and self.participant.vars["payoff1_self"]  ==10:
               return ['q0', 'q2',  'q_nochange']
            elif self.player.store_time == 0 and self.participant.vars["payoff1_self"] == 1:
               return ['q0', 'q2',  'q_change']
            else:
                pass

    def before_next_page(self):
        self.player.set_payoffs()
        self.player.set_bonus()
        self.player.set_payoff3()

My HTML Survey page simply contains `{% formfields %}`to fetch the dynamic fields.  If instead of using `get_form_fields()` I simply specify a form field in `form_fields=[ ]` , the error disappears. Anyone knows what is going on here?

It could be because some branches of your get_form_fields don't return any value. Try putting 'return []' at the very end of the code.

Writting what follows solved the error but did not solve the issue, now I am left with no formfields. Is this what you mean? I might've misunderstood you.

    def get_form_fields(self):
        if self.player.treatment == "treatment2":
        .
        .    
        .
        elif self.player.treatment == "treatment1":
         
        .
        .
        .
        return []