After installing the 4.7.21 upgrade my D6 site shows as upgraded and Civi appears to run correctly. However when I run the upgrade script I get error: Sorry but we are not able to provide this at the moment. DB Error: no such field""

If I delve into the error I find: [nativecode=1054 ** Unknown column 'civicrm_custom_group.is_public' in 'field list']"]

Can anyone guide me how to fix this?