sqlite fts4 can not use offsets funtion when use group by

I have two table: table 1: fts virtual table

docid Long auto increase, content Text, (fts column)

table 2: meta data table

docid Long(foreigner key of table1 docid), username Text, timestamp Long

sql: select table1.docid content, username, MAX(timestamp) as time, offsets(table1) from table1, table2 where content Match "a" AND table1.docid = table2.docid group by username order by timestamp desc limit 3;

this sql will not execute, but i delete "group by username", it can execute. why?