Django Forms - DateInput not populating from instance

I'm trying to set up an edit form for a Django model which includes a DateField. I've set this field as a forms.DateInput in forms.py. This works fine for creating a new instance of the model, but when I try to populate the form with an existing instance the DateInput field remains blank even though all of the other fields are populated correctly.

If I revert to the default TextField input then the data is recalled correctly. I've also tried to set a format in the DateInput widget.

models.py

class Rider(models.Model):
    first_name = models.CharField(max_length=40)
    surname = models.CharField(max_length=40)
    MALE = 'M'
    FEMALE = 'F'
    GENDER_CHOICES = [
        (MALE, 'Male'),
        (FEMALE, 'Female'),
    ]
    gender = models.CharField(max_length=1, choices=GENDER_CHOICES)
    dob = models.DateField("Date of Birth", auto_now = False, auto_now_add = False)
    club = models.CharField(max_length=50,blank=True, null=True)
    bc_number = models.IntegerField("British Cycling Membership Number", blank=True, null=True)
    linked_account = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE, default=1)

views.py

def rider_edit(request, pk):
    rider = get_object_or_404(Rider, pk=pk)
    if request.method == "POST":
        form = RiderForm(request.POST, prefix='rider', instance=rider)
        if form.is_valid():
            rider = form.save(commit=False)
            rider.linked_account = request.user
            rider.save()

            return redirect('rider_list')
    else:
        form = RiderForm(prefix='rider', instance=rider)
    return render(request, 'riders/rider_new.html', {'riderform': form})

form.py

from django import forms
from .models import Rider, MedicalInfo


class RiderForm(forms.ModelForm):

    class Meta:
        model = Rider
        fields = ('first_name', 'surname', 'gender', 'dob', 'club', 'bc_number')
        widgets= {
            'dob': forms.DateInput(attrs={'type': 'date'}, format='%d/%m/%Y')
        }

rider_new.html

<h2>New Rider</h2>
    <form method="POST" class="post-form">
        {% csrf_token %}
        {{ riderform.as_p }}
        <button type="submit" class="save btn btn-default">Add Rider</button>
    </form>

The dob field is the only field that doesn't populate correctly from the database, it should show a date in the format dd/mm/YYYY e.g. "10/09/2010". It's actually showing the default "dd/mm/YYYY".