Adding a user to the Exception Users list fails in vSphere 6.5

Hi,

Has anyone ran into problems when trying to add users to the lockdown mode exception users? I've tried this in my lab - vCSA 6.5 managing ESXi 6.5 hosts (all latest) and I keep getting a "The user or group named 'vsphere.local\Jason' does not exist." when selecting an account from the SSO domain user list.

Thanks

Jason