Error while enabling 4.0.x-dev in D9-RC1

Issue #3143960 on drupal.org by wrd.

I get the following error when enabling DRD-4.0.x-dev in Drupal 9 RC1:

[Fri May 29 11:57:52.808491 2020] [fcgid:warn] [pid 923:tid 123145535488000] [client 127.0.0.1:55212] mod_fcgid: stderr: Uncaught PHP Exception Drupal\\Core\\Config\\Schema\\SchemaIncompleteException: "Entity type 'Drupal\\Core\\Config\\Entity\\ConfigEntityType' is missing 'config_export' definition in its annotation" at /Users/dicksb/Sites/devdesktop/9.0/httpdocs/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php line 251, referer: https://drupal90.dd:8443/admin/modules/list/confirm?filter=did

There's nothing here that tells me it's actually a DRD problem, so maybe it's not -- but as enabling the module was the only change I made, it seemed the likeliest reason.