Column 'displaygroupid' in field list is ambiguous in showthread.php

[bobdell01]

If someone could help me, I sure would appreciate it.

I added a column to table post: post.to_user (btw, values are stored correctly)
which is a userid

I added this plugin to hook: showthread_query

$hook_query_fields = ",user.username AS to_username";
$hook_query_joins = "LEFT JOIN " . TABLE_PREFIX . "user AS u ON(u.userid = post.to_user)";

The line:

$hook_query_joins = "LEFT JOIN " . TABLE_PREFIX . "user AS u ON(u.userid = post.to_user)";

Seems to be causing the error: "Column 'displaygroupid' in field list is ambiguous"

I previously ran the following query in phpMyAdmin, and got the correct results:

SELECT user.username AS to_username
FROM post
LEFT JOIN user ON ( user.userid = post.to_user )
WHERE post.postid = 173

What am I doing wrong?

(I'm running 3.6.5)