Simultaneous subscriptions on one publication

I have a collection containing messages and comments made available through a publication that takes several arguments. When I subscribe to the publication in two different templates, the messages load while the comments do not.

Here is my publication:

Meteor.publish("posts", function (category, domain, email, commentParent, limit) {
    if (category == "anonymous") {
        return Posts.find({category: category, domain: domain, reportedBy: {$ne: email}, reports: {$lt: 3}, commentParent: commentParent}, {fields: {owner:0}, sort: {createdAt: -1}, limit: limit});
    } else {
        return Posts.find({category: category, domain: domain, reportedBy: {$ne: email}, reports: {$lt: 3}, commentParent: commentParent}, {sort: {createdAt: -1}, limit: limit});
    }
});

Here is the first subscription, which is working as intended:

Template.messages.onCreated(function() {
    var self = this;
    self.ready = new ReactiveVar();
    self.autorun(function() {
        var category = FlowRouter.getParam('category');
        var email = Meteor.user().emails[0].address; 
        var domain = String(email.match(/@(.*)$/g));
        var commentParent = {$exists: false};
        var handle = PostSubs.subscribe('posts', category, domain, email, commentParent, 10);
        self.ready.set(handle.ready());
    });
    Session.set('sort', 'new');
});

And here is the second subscription, which is not working:

Template.post.onCreated(function() {
    var self = this;
    self.ready = new ReactiveVar();
    self.autorun(function() {
        var category = FlowRouter.getParam('category');
        var email = Meteor.user().emails[0].address; 
        var domain = String(email.match(/@(.*)$/g));
        var commentParent = this._id;
        var handle = PostSubs.subscribe('posts', category, domain, email, commentParent, 10);
        self.ready.set(handle.ready());
    });
});

As you can see, the two subscriptions differ only in the commentParent (messages using { $exists: false } and comments using their parent ID). I have both the messages and post templates on the same page. I am able to see my messages, but not the comments.

I apologize if my code is not structured well - I am a novice. Would anybody be able to point me in the right direction?

Sorry for the double post. I’ve narrowed down the issue to the commentParent argument in my publish function. Something about having the following lines in my two subscriptions is causing the error:

subscription 1:
var commentParent = {$exists: false};

subscription 2:
var commentParent = this._id;

Stack overflow link: https://stackoverflow.com/questions/37307988/issue-with-multiple-subscriptions-on-one-publication