If the CK is on any firmware between 0.8.1 to 0.8.4 you need to update the *ubnt-tools* to 0.9.7-1 prior to attempting upgrade of the CK's firmware to 0.8.7.
1) SSH into the CK.
2) First check current ubnt-tools version
'dpkg -l |grep ubnt-tools'
this will show the current version most likely 0.9.5 or earlier
3) To upgrade the *ubnt-tools* to 0.9.7-1
'apt-get update && apt-get install --only-upgrade ubnt-tools'
4) Then update to the latest firmware(current at Jan18)
'ubnt-systool fwupdate https://dl.ubnt.com/unifi/cloudkey/firmware/UCK/UCK.mtk7623.v0.8.7.4158d32.171130.1346.bin'
this will upgrade the firmware to 0.8.7 and controller to 5.6.26(bundled with firmware)
5) If the update fails due to lack of free space issue the following commands:
`apt-get update ; apt-get install aufs-tools'
`/sbin/auplink / flush`
And then go back to step 4)