Function key_invalid 

pub fn key_invalid(
    ctx: Context,
    node_type: &str,
    op: &str,
    store: &str,
) -> PluginExecutionError

A key template that rendered to nothing.

{prefix}:kv: is still inside the namespace, so this is not a collision – but it would put every request on one shared key, turning a template typo into a cross-tenant leak. It fails loudly instead, with a 500: this is a policy-authoring/config fault, not an outage.

Only ever checked on the redis-backed data path (rendering a key is pointless without a backend to read or write it against), so this is #[cfg(feature = "redis-store")] like the rest of that path.