Getting "This QueryDict instance is immutable" on one Django view but not another

Getting "This QueryDict instance is immutable" on one Django view but not
another

I have two views, one that is working fine when I try to add to the POST
data when a form is not valid, another that gives me a "This QueryDict
instance is immutable" error when trying to do the same thing. I know
these views are too similar to exist in the first place and plan on
combining them... but would like to first understand what is the
difference that makes one of them fail.
This view works fine:
@login_required
def url_parse(request):
cu = request.user
if request.method == 'POST':
form = UrlBragForm(request.POST, request.FILES)
if form.is_valid():
t = handle_image_upload(form.cleaned_data['image'],cu.pk,'url')
if t:
b = Brag()
b.user = cu
b.image = t
b.url = form.cleaned_data['url']
b.description = form.cleaned_data['brag']
b.active = True
b.timestamp = time.time()
b.save()
tags = parse_tags(form.cleaned_data['brag'])
if tags:
for tg in tags:
b.tags.add(tg)
else:
errors = form._errors.setdefault("image", ErrorList())
errors.append(u"There was an issue with your image.
Please try again.")
else:
clean = cleanMessage(request.POST['brag'])
if clean != 'is_clean':
request.POST['brag'] = clean
else:
form = UrlBragForm()
return render_to_response('brag/url_brag.html', {'form':form,},
context_instance=RequestContext(request))
But this view gives me a "This QueryDict instance is immutable" when
trying to fill the request.POST['brag'] with the 'clean' data when view is
not valid:
@login_required
def brag_add_image(request):
cu = request.user
if request.method == 'POST':
form = ImageAddBragForm(request.POST, request.FILES)
if form.is_valid():
t = handle_image_upload(form.cleaned_data['image'],cu.pk,'url')
if t:
b = Brag()
b.user = cu
b.image = t
b.description = form.cleaned_data['brag']
b.active = True
b.timestamp = time.time()
b.save()
b.url = 'http://%s%s' %
(Site.objects.get_current().domain,
reverse('image_display', args=(b.pk,)))
b.save()
tags = parse_tags(form.cleaned_data['brag'])
if tags:
for tg in tags:
b.tags.add(tg)
else:
errors = form._errors.setdefault("image", ErrorList())
errors.append(u"There was an issue with your image.
Please try again.")
else:
clean = cleanMessage(request.POST['brag'])
if clean != 'is_clean':
request.POST['brag'] = clean
else:
form = ImageAddBragForm()
return render_to_response('brag/image_brag_add.html', {'form':form,},
context_instance=RequestContext(request))