way to custom sort column by fixed strings, then by field's content

I have a column that contains items like
'absolute root'
'root 3'
'root 4'
'root 5'
'scene 1'
'scene 2'
'scene 3'

and I would like them to sort in that order.

I tried:
select sti.description, sc.description from scene_thing_instances sti join
scenes sc on sti.scene_id = sc.scene_id
order by CASE sc.description
when (sc.description = 'absolute root'::text) then 1
when (sc.description ilike 'root%') then 2
else 3
END;

I was starting with this, and was going to add perhaps another case
statement.

But it gives me:
ERROR: operator does not exist: text = boolean
LINE 3: when (sc.description = 'absolute root'::text) th...
^
HINT: No operator matches the given name and argument type(s). You might
need to add explicit type casts.

I don't understand this because description is a text column, not boolean,
and certainly 'absolute root'::text is a text string.

This is 9.2.

Ideas, anyone?

Thanks,
Susan