Can't turn on encryption on existing Time Machine backup

When I try to turn on encryption on an existing unencrypted Time Machine Backup — following Apple's instructions — I get the following error:

There was an error preparing the Time Machine backup disk “Time Machine”.

POSIX reports: The operation couldn’t be completed. Invalid argument.

Specifications:

• MacBook Pro (13-inch, mid 2012), 16 GB memory.
• macOS 10.13.1
• MacBook Pro storage is an SSD formated as: APFS (Encrypted).
• backup is over USB 3.0 to an external 2.5" HDD.
• external HDD backup drive is formated as: Mac OS Extended (Journaled).

Any suggestion what I could try, in order to turn my working, existing and unencrypted Time Machine backup into an encrypted Time Machine backup?

Thanks