Count(*) with order by not working on PostgreSQL which works on Oracle

Below is the Sql query which works on oracle but not working on PostgreSQL.

select count(*) from users where id>1 order by username;

I know that order by has no meaning in this query but still why it's working on oracle. Below is error on PostgreSQL

ERROR: column "users.username" must appear in the GROUP BY clause or be used in an aggregate function
Position: 48

SQLState: 42803

PostgreSQL version 9.6.3