Retrieve Fb post comments using FBGraph - facebook-graph-api

Question

I am trying to retrieve Fb comments on a particular post. I tried to implement following method:
FbGraphResponse *fb_graph_response = [fbGraph doGraphPost1:#"3788106577940/comments" withPostVars1:[[NSDictionary alloc]initWithObjectsAndKeys:#"Hello",#"Hi", nil]];
SBJSON *parser = [[SBJSON alloc] init];
NSDictionary *facebook_response = [parser objectWithString:fb_graph_response.htmlResponse error:nil];
//[parser release];
//let's save the 'id' Facebook gives us so we can delete it if the user presses the 'delete /me/feed button'
self.feedPostId = (NSString *)[facebook_response objectForKey:#"id"];
NSLog(#"feedPostId, %#", feedPostId);
But i am getting the following error:
<CFBasicHash 0x6a64360 [0x1751b38]>{type = mutable dict, count = 1,
entries =>
11 : <CFString 0x6a64e30 [0x1751b38]>{contents = "error"} = <CFBasicHash 0x6a64170 [0x1751b38]>{type = mutable dict, count = 3,
entries =>
2 : <CFString 0x6a64410 [0x1751b38]>{contents = "type"} = <CFString 0x6a645f0 [0x1751b38]>{contents = "OAuthException"}
3 : <CFString 0x6a64590 [0x1751b38]>{contents = "message"} = <CFString 0x6a644f0 [0x1751b38]>{contents = "(#1705) : You cannot make blank posts. Please enter some text and try again."}
6 : <CFString 0x6a64200 [0x1751b38]>{contents = "code"} = 1705
}
}
i have accessed following permissions:
[fbGraph authenticateUserWithCallbackObject:self andSelector:#selector(fbGraphCallback)
andExtendedPermissions:#"user_photos,user_videos,publish_stream,offline_access,user_checkins,friends_checkins,read_stream"];
Please do help.Thanks!!