Facebook ads 1634011 error: "Invalid property selector given action types"

I keep getting this error when I try to create an ad (adgroup) using the facebook ads python sdk. Here is the setup I have and the result I want to achieve:

• i already have a campaign (campaign group)


• i already have an adset


• the bid type I want to have is CPA


• i'm able to create the creative bound to the unpublished post


• BUT when i want to create the adgroup bound to the creative, i receive the error: Invalid property selector given action types. The property selectors don't map to any known objects based on the action type(s) in the spec.

I first thought that something wasn't created in the right way (for instance the post didn't exist). But I tripled checked with no success...

Here is the whole backtrace from the python client:

FacebookRequestError: Call was not successful 
Request:
    {'files': {}, 'path': 'http://ift.tt/1CLLcoO', 'params': {'name': 'SDK ad name', 'adgroup_status': 'ACTIVE', 'objective': 'WEBSITE_CLICKS', 'creative': '{"creative_id":"xxxxxxx"}', 'conversion_specs': '[{"action.type":"link_click","post.wall":xxxxxxxx,"post_id":"xxxxxxx"}]', 'campaign_id': u'xxxxxxxx'}, 'method': 'POST', 'headers': {'User-Agent': 'fb-python-ads-api-sdk-2.2.5'}}
Response:
    HTTP Status: 500
    Headers:{'content-length': '307', 'facebook-api-version': 'v2.2', 'expires': 'Sat, 01 Jan 2000 00:00:00 GMT', 'x-fb-debug': 'xxxxxxx', 'x-fb-rev': '1604454', 'connection': 'keep-alive', 'pragma': 'no-cache', 'cache-control': 'no-store', 'date': 'Thu, 19 Feb 2015 01:16:54 GMT', 'access-control-allow-origin': '*', 'content-type': 'application/json; charset=UTF-8', 'www-authenticate': 'OAuth "Facebook Platform" "invalid_request" "Invalid parameter"'}
    Body: {"error":{"message":"Invalid parameter","type":"FacebookApiException","code":100,"error_subcode":1634011,"is_transient":false,"error_user_title":"Invalid property selector given action types","error_user_msg":"The property selectors don't map to any known objects based on the action type(s) in the spec."}}

Im' a bit stuck here, does anyone have a clue?

Thanks :-)