Regular update of OS::SoftwareDeployments fails

For example, create a stack with this template:

heat_template_version: 2013-05-23
parameters:
    image:
        type: string
    network:
        type: string

resources:
    server:
        type: OS::Nova::Server
        properties:
            image: {get_param: image}
            flavor: m1.micro
            user_data_format: SOFTWARE_CONFIG
    my_depl:
        type: OS::Heat::SoftwareDeployments
        properties:
            config: {get_resource: config}
            servers: {'0': {get_resource: server}}
            signal_transport: NO_SIGNAL

    config:
        type: OS::Heat::SoftwareConfig
        properties:
            config: xxxxxxxxxxxxxxxxxxxxxx

update config, call stack update

This results to UPDATE_FAILED state of the stack.