Aggregate error: 'No items for key "%s" in group(s) "groupname".'

Aggregate item definition:

key_: grpsum["{$PG_HOST_GROUP}","psql.primary_server[{$PGSCRIPTDIR},$PGSCRIPT_CONFDIR}]",last]
error: No items for key "%s" in group(s) "Cluster: Postgres/GEN/test".

Source item definition:

key_: psql.primary_server[{$PGSCRIPTDIR},{$PGSCRIPT_CONFDIR}]

The user macro contents are also identical on the aggregate host and the source hosts, which are in the group "Cluster: Postgres/GEN/test".

The group name matches and all source items have valid values, but all aggregate items are 'not supported' with an error like this.

That "%s" in the error message should probably expand to something useful, but is the issue just one of error message parameter expansion or is it really comparing the key to a literal "%s" string and failing because of that..?