Can't reduce HDD size (thin provisioning)

Hello,

Running ESXi 6.5.0 version 650.9.6.0.28.

We have several VMs (Windows and Linux).

When trying to reduce the size of a hard disk, we get the message:

Enter a disk size which is larger than its original capacity.

Disks are thin provisioned.

Are we missing something or this is not possible with ESXi 6.5?

Thanks

Alex