Relock bootloader - "FAILED (remote: unknown command)"

I'm getting rid of my Sony XZ2 Compact. I just flashed the original firmware using Emma and now I'm trying to relock the bootloader. I've tried running fastboot oem lock and fastboot flashing lock both as a normal user and root with no avail. I always get the following error: >> fastboot oem lock ... FAILED (remote: unknown command) finished. total time: 0.000s When I run fastboot devices I can see my phone listed. USB-Debugging is active in the developer options. How can I lock the bootloader so that I don't get the screen with "*Your device software can't be checked for corruption. Please lock the bootloader.*" when booting the phone?