Content Index Unknown State after Exchange 2013 CU15 update

Hi guys - I have just upgraded to CU15 and when checking Get-MailboxDatabaseCopyStatus | FL Name,*Index* I receive the following:-

Name                         : MDB1\CERBERUS
ContentIndexState            : Unknown
ContentIndexErrorMessage     : Could not find registry value of Index Status for database
                               {2c569645-36e4-4906-848d-5c38d67c7574}.
ContentIndexErrorCode        :
ContentIndexVersion          :
ContentIndexBacklog          :
ContentIndexRetryQueueSize   :
ContentIndexMailboxesToCrawl :
ContentIndexSeedingPercent   :
ContentIndexSeedingSource    :
ContentIndexServerSource     :

I have tried all the usual fixes.  

Deleting content index folder and restarting Search services.

This has not resolved the issue, as I suspect the registry keys are the issue here.

When checking HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ExchangeServer\v15\Search\IndexStatus there are no keys present. 

This is a single Exchange 2013 server, so I do not have another server to copy the keys from.

Any ideas?