Why do I get an object is not iterable error?

Question

Why do I get the following error in my app

Caught TypeError while rendering: 'ModelNameHere' object is not iterable

but I don't get it when I execute it from the shell?

I just have a custom field in my form which inherits from forms.ModelForm

custom_serving_size = forms.ChoiceField(ServingSize.objects.all(), widget=forms.Select(attrs={'class':'ddl'}))

EDIT

This is my form class

class RecipeIngredientForm(forms.ModelForm):
    serving_size = forms.ChoiceField(choices=ServingSize.objects.all())

The error happens on ServingSize.objects.all()

Answer 1

custom_serving_size = forms.ChoiceField(ServingSize.objects.all(), widget=forms.Select(attrs={'class':'ddl'}))

this has to be

custom_serving_size = forms.ModelChoiceField(queryset=ServingSize.objects.all(), widget=forms.Select(attrs={'class':'ddl'}))

or

custom_serving_size = forms.ChoiceField(choices=[(obj.id, `text user sees`) for obj in ServingSize.objects.all()], widget=forms.Select(attrs={'class':'ddl'}))