# Mentions or AutoComplete do not work as expected - You receive a java.lang.NullPointerException: userkey

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## Symptoms

When attempting to mention a user, you are not able to click the user name to insert, or the search does not work as expected to find a user. You may also find this behaviour when working with other areas of Confluence that ask you to select a User.

The following appears in the atlassian-confluence.log (You may also see a similar stack trace in the console of your browser):

 -- url: /rest/prototype/1/search.json | userName: userName | referer: http://wiki/some/path/
java.lang.NullPointerException: userkey
at com.atlassian.sal.api.user.UserKey.<init>(UserKey.java:27)

## Cause

The index is damaged or incomplete; and must be rebuilt.

## Resolution

Because you'll need to shut down Confluence in order to re-build the content indices, you may wish to schedule the downtime outside your normal operating hours.

Always ensure you have a full back up of your Home Directory before making changes to it.