Multiple Errors in Event Log Event ID 7182 and 41315

Recently I have noticed multiple Errors in the event log occurring every few minutes.

Event 7182 Web Application Index Search failed and Event 41315 Index Query Server A search failed with error "Value cannot be null. Parameter name: key"

I've run a verification on the Indexes for the users whose mailboxes this is effecting but this didn't highlight any problems.

Has anyone come across this before, are there any good articles for diagnosing the issue.