Invoke Cirrus.Settings DecryptData via SWIS v3 over HTTPS

Hi  - I am trying to Invoke a SWIS Verb via HTTPS using the following curl command:

curl -k -u <user>:<pass> -X POST -H "Content-Type: application/json"  -d '["ABC"]' https://<host>:17778/SolarWinds/InformationService/v3/Json/Invoke/Cirrus.Settings/DecryptData

I can not find any general documentation on how to present Verb arguments over HTTPS, so I am assuming the single string argument accepted by DecryptData should be posted as a string within a JSON array?

However the response I get is:

{"Message":"Verb Cirrus.Settings.DecryptData: Cannot find assembly"}

The Verb invocation works as expected when tested with the SwisInvoke.exe tool provided in the Orion SDK.

Does this look like a problem with the Information Service install or should I be calling the Verb with a specific argument format?

Thanks

J