Statistics page on MediaWiki displaying an error

The link above displays the error:

A database query syntax error has occurred. This may indicate a bug in the software. The last attempted database query was:
(SQL query hidden)
from within function “SiteStatsUpdate::cacheUpdate”. MySQL returned error “1054: Unknown column ‘ss_active_users’ in ‘field list’ (”.

I’m not sure how to fix it, or why it is happening.

It does not display on: