Sitecore EXM - upgrade Cannot use DataAdapterProvider as xDB is disabled

During the upgrade of Email Experience Manager through the versions, the following error was appearing after installing version 3.5

Cannot use DataAdapterProvider as xDB is disabled.

During the upgrade of the core Sitecore product,  two settings were set to false inside the Sitecore.Xdb.Config file. Setting these back to true and the site will load as expected.

1. Xdb.Enabled
2. Xdb.Tracking.Enabled

In my case, email experience manager was the last piece of the upgrade puzzle, so these could be enabled again anyway.