Why Graph API Explorer doesn't return the wall 'feed' based on a group's id

When I test the Graph API Explorer from facebook site and write the following parameters:

https://graph.facebook.com/'groupid'/feed?access_token='my access token'

it returns

{
  "data": 
 [
 ]
}

I tried it with several group ids and all return the same result, while if I try members instead of feed it returns the list of members.

Is there a bug in the feed of a group? Here is how you can generate the issue:

1. Get an access token: xxxxx
2. Get any Group ID: 123456

3. Make the graph api explorer call to retrieve the group's wall based on the group id

   https://graph.facebook.com/123456/feed?access_token=xxxxxxx

The thing is I first thought the problem was from my code, but when I tried it from facebook graph api explorer I found out that its not returning any results. Please I would really appreciate the help as this is part of my master's project and the due is within two weeks!!