Detailed information on how to use tmutil inheritbackup

I just got a new iMac (2020). It is running the latest Big Sur distribution. I have TM backing up to an external drive off a MacBook running Big Sur. The external drive is formatted Mac OS Extended (Case-sensitive, Journaled). I want to inherit the backup from the Macbook to the iMac, and to remain external on the same drive. When I run:

sudo tmutil inheritbackup /Volumes/Apple\ Backup/Backups.backupdb/MAC<machine_id>

I get the following error (which I did not get the last time I ran tmutil inheritbackup from the previous machine):

Failed to inherit machine store at '(null)', error: Error Domain=TMStructureErrorDomain Code=7 "Structure has wrong type" UserInfo={NSLocalizedDescription=Structure has wrong type}

I need more detailed information on why this is happening and how to fix it. I do not want to start a new backup on the same device. Thanks for any help.