PFQuery whereKey doesNotMatchKey inQuery

Question

I have 2 tables like so:

-  Posts
    -  objectId (default objectId)
    -  content (text)
-  Comments
    -  objectId (default objectId)
    -  post (pointer to Posts)
    -  text (text)

I've been trying to grab Posts that don't have comments.

Here's what I've tried so far:

PFQuery *query_comments     =   [PFQuery commentQuery];
[query_comments whereKeyExists:@"post"];
[query_comments selectKeys:@[@"post"]];

PFQuery *query_posts        =   [PFQuery postsQuery];
[query_posts whereKey:@"objectId" doesNotMatchKey:@"post" inQuery:query_comments];
[query_posts findObjectsInBackgroundWithBlock:^(NSArray * _Nullable objects, NSError * _Nullable error) {

    printf("\n\n");
    // objects should now contain all Posts that don't have comments

}];

Edit

Currently, the query_posts ignores the query_comments and just fetches posts like normal (as if the query_comments didn't exist).