DocumentForm save error: object has no attribute 'cleaned_data'

[denger]

pip freeze

(deco_env)➜ speechtv pip freeze
Django==1.4.2
mongodbforms==0.3
mongoengine==0.8.3
pymongo==2.7.1

Form

class SpeechTVForm(DocumentForm):
    class Meta:
        document = SpeechTV
        fields = ['source_link', 'embed', 'share_author']

SpeechTV Document

class SpeechTV(Document):
    title = StringField(max_length=30)
    source_link = StringField()
    embed = StringField()
    intro = StringField()
    share_author = StringField()
    ...more...

view.py

def share(request):
    if request.method == 'POST':
        form = SpeechTVForm(request.POST) 
        form.save()

when i post form, show this error message:

Request Method: POST
Request URL: http://127.0.0.1:8000/submit_speech.tv
Django Version: 1.4.2
Python Executable: /Users/denger/Workspaces/Works/deco_workspace/deco_env/bin/python
Python Version: 2.7.3
Python Path: ['/Users/denger/Dropbox/Codes/speechtv',/Users/denger/Workspaces/Works/deco_workspace/deco_env/lib/python2.7/site-packages/PIL']
Installed Applications:

Traceback:
File "/Users/denger/Workspaces/Works/deco_workspace/deco_env/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
111 . response = callback(request, _callback_args, *_callback_kwargs)
File "/Users/denger/Dropbox/Codes/speechtv/speechtv/views.py" in share
26 . form.save()
File "/Users/denger/Workspaces/Works/deco_workspace/deco_env/lib/python2.7/site-packages/mongodbforms/documents.py" in save
533 . fail_message, commit, construct=False)
File "/Users/denger/Workspaces/Works/deco_workspace/deco_env/lib/python2.7/site-packages/mongodbforms/documents.py" in save_instance
178 . instance = construct_instance(form, instance, fields, exclude)
File "/Users/denger/Workspaces/Works/deco_workspace/deco_env/lib/python2.7/site-packages/mongodbforms/documents.py" in construct_instance
98 . cleaned_data = form.cleaned_data

Exception Type: AttributeError at /submit_speech.tv
Exception Value: 'SpeechTVForm' object has no attribute 'cleaned_data'
Request information:
GET: No GET data

POST:
source_link = u'sfasfsafsafasf'
csrfmiddlewaretoken = u'ppHzHO8TREgHfKBGobdk7UlW7OvFnICA'
embed = u'afsdfasfasd'
share_author = u'afd'