I was wondering if anyone has had any experience assigning a Topic to a record from the Question object before.

Some background
We are currently buiding a custom site using VisualForce and custom JS to essentially provide our own interface to the standard SF community functionality.

The page I'm currently working on is our Q&A page - specifically the page through which customers can post their own Questions to our customer community.

The Goal
The issue is, we need to be able to filter our questions by topic - not just by data category.  This must also be achieved purely in APEX without any standard apex components on the VF page.

What has been done so far
So far, I have enabled topics for the question object (under Topics for Objects in SF setup).  I have enabled system permissions for the appropriate profiles to create, edit, and assign topics.  Yet I keep running into an apex exception when I try to assign the topic to the question.

Attempt 1: Manually insert a record into the TopicAssignment object
In the code, I create and insert a question (and topic if required).  This works without any problems.  Then I run the following:

TopicAssignment a = new TopicAssignment(TopicId=t.Id, EntityId=q.Id, NetworkId=n.Id);
insert a;

This line will throw the following error:

Insert failed. First exception on row 0; first error: FIELD_INTEGRITY_EXCEPTION, Entity ID: id value of incorrect type: 906N00000004EF7IAM: [EntityId]

Attempt 2:  Use the ConnectApi
I then found a new class in the ConnectApi called Topics which does not yet appear to be documented.

ConnectApi.Topics.assignTopic(networkId question.Id, topic.Id);

Using this instead yields the following error:

Failed to assign topic to 906N00000004EFC. Please ensure you have permissions to assign topics for this record.

Both of the above errors still occur if the class has sharing off - e.g.

public without sharing class ClassName {
}

I am hoping to achieve this without having to create a new object as a workaround.

So, I was wondering if anyone else has had this issue?  Has anyone succesfully assigned a topic to a question in pure APEX?  Can anyone help?

Thank you very much,

Nic