Cannot add flag link in a comments view

Steps to reproduce:

1. Add new flag of type "comments"
2. Add new view that shows comments. Display format: fields
3. Inside the view: add new relationship: flags: "comments".
4. Inside the view: add new field: flag: flag link

I'm getting this error:

An AJAX HTTP error occurred.
HTTP Result Code: 500
Debugging information follows.
Path: /admin/structure/views/view/commentstest2/preview/page/ajax
StatusText: Internal Server Error
ResponseText:

I get a white screen when I go to the views page.
No dblog error.

Versions:

• d7.12
• flag 7.x-2.0-beta
• views 7.x-3.2
• ctools 7.x-1.0-rc1

No other modules installed.

I'm not getting any error when the view shows nodes, instead of comments.

Thanks for your help!