Error when trying to remove a license

Hi everyone, hoping for some help with this one.

I'm trying to remove a license from a user in my tenant: o365_user1@<domain>.com using PowerShell.

Using Get-MsolUser I can confirm that the ENTERPRISE PACK (E3) and EMS (Enterprise Mobility + Security E3) licenses are assigned. I can also verify in the web portal. Not all switches are activated for the E3:

However when I run Set-MsolUserLicense to try to remove the license, I get an error, even though as far as I can tell, I'm following the correct syntax according to https://docs.microsoft.com/en-us/powershell/module/msonline/set-msoluserlicense?view=azureadps-1.0. The error occurs on the ENTERPRISEPACK license removal - the EMS removal goes through fine.

I've tried this on two different accounts without success. 

I don't think it's a permissions issue as I've been able to use my credentials for Set-MsolUserLicense -AddLicenses on these accounts.

Not sure it's relevant, but my long term goal is to pipe a CSV into a script to be able to remove existing E3 and M+S licenses and replace with E1s/F1s with Exchange Online Plan 2 licenses. I've got that second part working reliably in a script, but can't figure out why the license removal is failing.

Any help appreciated!