Unable to change active slot using fastboot

[shodan8192]

Hi everyone, I'm using fastboot 1:7.0.0+r33-2 (latest version in repo) on Ubuntu 18.04 and i can't change active slot :

$ fastboot getvar current-slot
current-slot: _b

$ fastboot set_active a
Slot a does not exist. supported slots are:


(just empty lines)

Bootloader is unlocked.

Can anyone confirm that there is a problem with that particular version ? I can't check with the one from SDK Platform Tools at the moment - maybe it's OK with it.
BTW, it is possible to switch slot with locked bootloader ?

 

[honem223]

Bootloader Version...: 00872-LAHAINA-2.487493.1
Baseband Version.....: 00781-LC_ALL_PACK-1
Serial Number........: f2e1c7ed
--------------------------------------------
extracting android-info.txt (0 MB) to RAM...
Checking 'product' OKAY [ 0.000s]
Setting current slot to 'b' FAILED (remote: 'Slot Change is not allowed in merging state')
fastboot: error: Command failed


Please help me