How to get LAST_VALUE in a SESSION window in FlinkSQL?

I am using session windows in Flink SQL (1.13). Is there a way (must be in SQL, no UDFs etc.) to get the last value of a certain field (in other words: this would be a value at window_end)? I was trying with:

SELECT user_account_id, 
       SESSION_START(request_timestamp, INTERVAL '30' MINUTE) AS window_start, 
       SESSION_END(request_timestamp, INTERVAL '30' MINUTE) AS window_end,
       LAST_VALUE(package)
GROUP BY SESSION(request_timestamp, INTERVAL '30' MINUTE), user_account_id

but I am getting error:

Could not find an implementation method 'merge' in class 'org.apache.flink.table.planner.functions.aggfunctions.LastValueAggFunction' for function 'LAST_VALUE' that matches the following signature:
void merge(org.apache.flink.table.data.RowData, java.lang.Iterable)

I guess that using window functions (OVER(...)) would not work here. Any hints appreciated!

Solution

I've got the answer that this is not supported yet. A workaround would be to create a custom user-defined aggregate function (UDAGG) . Apart from that, there is a new Jira for that functionality.