Cannot use discovery key ({#CLIENT}) as part of Calculated item prototype Key.
Error looks like this:

Invalid key "{#CLIENT}_err_rate" for item prototype "{#CLIENT} nginx error rate" on "cdnnow-edge-client-template": incorrect syntax near "{#CLIENT}_err_rate".

Also tried:

"{#CLIENT}"
"\#\{CLIENT\}"
({#CLIENT})
"({#CLIENT})"
"(\{\#CLIENT\})"