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

[Susan Cassidy <susan.cassidy@xxxxxxxxxxxxxxxxxxxxxxxx>]

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