Using Python & XML-RPC to add a custom field to a Wordpress post?

Question

I want to add an 'enclosure' custom field to an existing Wordpress post using Python & XML-RPC.

My code looks like this:

def add_enclosure(server, post_id, enclosure):
    post_data = server.metaWeblog.getPost(post_id, username, password)
    custom_fields = post_data['custom_fields']
    new_id = max([int(field['id']) for field in custom_fields]) + 1

    custom_fields.append({'id': "%s" % (new_id), 'key': 'enclosure', \
                            'value': "%s\n%s\n%s" % \
                            (enclosure['url'], enclosure['length'], enclosure['type'])})
    server.metaWeblog.editPost(post_id, username, password, \
                                {'custom_fields': custom_fields})

But I get the following error:

xmlrpclib.Fault: <Fault 500: 'Sorry, your entry could not be edited. Something wrong happened.'>

What am I doing wrong?