Marking collapsed item as read fails

[dansanders]

Steps to reproduce:

1. Select a collapsed item by pressing 'n'
2. Mark it as read by pressing 'm'

Actual result:
The item's background turns grey, but it is not marked as read. The HTTP request to do so fails with a 500 Internal Server Error (response body is "invalid character 'e' in literal false (expecting 'a')").