Updating pg_attribute - Permission denied - Mailing list pgsql-general

I was trying to update attnotnull = 't' in the pg_attribute to add Not Null
constraint to a table.  It gave me a Permission denied.

Is this standard? Am I not allowed to change system catalogues (I am using
the postgres superuser!)  I thought I had heard this would work.  Or can you
only change attnotnull to false i.e. remove a not null constraint?  I
suppose this would make some kind of sense, as there could already be Nulls
in the field.

Thanks in advance

Ben