sort Observation by date gives an "Unsupported search type" error

[ren...@gmail.com]

I tried to sort observation by
 
SELECT fhir_search('{"resourceType": "Observation", "queryString": "_sort:asc=date"}')

but got the error,

ERROR:  Error: Unsupported search type [undefined]}
DETAIL:  plv8_init() LINE 2925:       throw new Error("Unsupported search type [" + searchType + "]}");

not sure why since searchparamaters table has a entry for Observation-date


Rama