What does an exitcode of null from a script imply?

When using the Script plugin, I usually see an exitcode =-999 if the script cannot be executed (e.g. its not found).

Executing the same Script plugin using the Execute operation and the same arguments as on the Availability check, returns the expected output. But those same arguments executed on the Availability scan, returns the following. Has it timed out?

CLI results: exitcode=[null]; error=[null]; output=<cr><lf>