Redshift pg_user table throws Invalid Operation error on JOIN

when I run the following query,

select * from stl_query q
join pg_user u on q.userid = u.usesysid
where u.usename = 'admin';

I get the following error:

SQL Error [500310] [0A000]: [Amazon](500310) Invalid operation: Specified types or functions (one per INFO message) not supported on Redshift tables.;

The query is run on the leader node. What am I doing wrong?