Error 69808 converting disk using CoreStorage

I'm trying to convert a disk to encrypted CoreStorage, using the following terminal command: diskutil cs convert /dev/disk1s1 -passphrase 'password goes here'

however I receive and error "error -69808: Some information was unavailable during the internal lookup"

Here's the terminal output once I run the command:

Started CoreStorage operation on disk1s1 Reptar

Resizing disk to fit Core Storage headers

Creating Core Storage Logical Volume Group

Attempting to unmount disk1s1

Switching disk1s1 to Core Storage

A problem occurred; undoing all changes

Undoing creation of Logical Volume Group

Reclaiming space formerly used by Core Storage metadata

Error: -69808: Some information was unavailable during an internal lookup

Anyone come across this error before? My googling has only found a few instances of the error but none are related to this specific command

Thanks!