How can I encrypt a Disk Utility RAID volume?

I have two SSDs in an external enclosure, exposed to the OS as independent drives. I want them to appear as a single encrypted APFS volume, with RAID 1 mirroring. I'd like to set up RAID via the native Disk Utility, rather than third-party software like SoftRAID. In Disk Utility, I can create a RAID 1 set containing the two drives. However, in the format choices I only see APFS, not APFS (Encrypted). If I create the RAID Set using APFS, right-click the volume in Finder and choose "Encrypt", the operation succeeds, and the volume works. However, Disk Utility immediately shows the RAID Set as "Offline" with one disk "Missing/Damaged". The same thing happens if I use Disk Utility to Erase the volume as APFS (Encrypted). Is there any way to encrypt the volume without breaking the RAID Set?