[Python] Why I get "KeyError" exception in django instead of "This field is required"...

I'm new to Django, I have a registration form, Everything works fine If I fill all fields and when I don't fill all the fields. But when I submit a form without a username field I get a "Key Error" instead of "This field is required" Exception since I have declared the field is required on the form class.

forms.py

class UserRegistration(forms.ModelForm):
first_name = forms.CharField(label='First Name', max_length=50)
last_name = forms.CharField(label='Last Name', max_length=50)
username = forms.CharField(label='Username', max_length=50)
email = forms.EmailField(label='Email', max_length=50)
password = forms.CharField(label='Password', widget=forms.PasswordInput, max_length=50, validators = [validators.MinLengthValidator(6)])
password2 = forms.CharField(label='Repeat Password', widget=forms.PasswordInput, max_length=50)

class Meta:
model = User
fields = ('username', 'first_name', 'last_name', 'email', 'password', 'password2')

def clean_username(self):
username = self.cleaned_data['username']
email = self.cleaned_data['email']

if username and User.objects.filter(username=username).exclude(email=email).count():
raise forms.ValidationError('This username address is already in use.')
return username

def clean_email(self):
email = self.cleaned_data['email']
username = self.cleaned_data['username']

if email and User.objects.filter(email=email).exclude(username=username).count():
raise forms.ValidationError('This email address is already in use.')
return email

def clean_password(self):
password = self.cleaned_data['password']
if len(password) < 6:
raise forms.ValidationError("Password is too short")
return password

def clean_password2(self):
cd = self.cleaned_data
if cd['password'] != cd['password2']:
raise forms.ValidationError('Passwords don\'t match.')
return cd['password2']


views.py

def register(request):
if request.method == 'POST':
form = UserRegistration(request.POST)
if form.is_valid():
new_user = form.save(commit=False)
new_user.set_password(
form.cleaned_data.get('password')
)
new_user.save()
return render(request, 'authapp/register_done.html')
else:
return render(request, 'authapp/register.html', {'form':form})
else:
form = UserRegistration()

context = {
"form": form
}
return render(request, 'authapp/register.html', context=context)


Error Traceback Error Page Screenshot

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