vfs.fs.discovery fails with error 'Value should be a JSON object'

I have filesystem LLD setup using the vfs.fs.discovery key in a standard template (one for solaris, one for linux)

This LLD rule fails on hosts monitored via a proxy, the same rule (using same template) works fine if monitored directly. The error shows up in the Error column of the hosts Discovery page.

I can do a zabbix_get -s <host> -k vfs.fs.discovery from the proxy and it returns valid data.