count rows from a derived table does not like count(der_tbl.*)

Database
Enthusiast

count rows from a derived table does not like count(der_tbl.*)

-- doesn't work
SEL COUNT(a.*) l
FROM (SEL * FROM dbname.tablename) a

--does work
SEL COUNT(a.columnname) l
FROM (SEL * FROM dbname.tablename) a

--any anybody provide a technical explanation as to why the above fails??