coalesce view error

hi, i have this view defined as:

create view mikeotest as
select
coalesce(topflow_application.rpt_name,topflow_application.tfap_name) AS
ipd_desc,
graphics_03.ipd_date,
graphics_03.day,
graphics_03.gr_bill_amt_total,
graphics_03.gr_byte_qty_total,
graphics_03.gr_ipd_sessions
FROM graphics_03, topflow_application
WHERE (graphics_03.ct_key = topflow_application.ib_ct_key
or graphics_03.ct_key = topflow_application.ob_ct_key);

which postgres generates this from:

SELECT CASE WHEN (topflow_application.rpt_name NOTNULL)
THEN topflow_application.rpt_name
WHEN (topflow_application.tfap_name NOTNULL)
THEN topflow_application.tfap_name
ELSE NULL::unknown
END AS ipd_desc,
graphics_03.ipd_date, graphics_03."day",
graphics_03.gr_bill_amt_total,
graphics_03.gr_byte_qty_total, graphics_03.gr_ipd_sessions
FROM graphics_03, topflow_application
WHERE ((graphics_03.ct_key = topflow_application.ib_ct_key)
OR (graphics_03.ct_key = topflow_application.ob_ct_key));

when i try to run this query against this view, i get the following:

select sum(gr_bill_amt_total), ipd_desc from mikeotest group by ipd_desc;
ERROR: Unable to identify an equality operator for type 'unknown'

i've played with the postgres definition changing NULL::unknown to 'stuff'
and other variations without success. any help would be appreciated.

the underlying table looks like this and has 2200+/- rows in it:

Table "graphics_03"
Attribute | Type | Modifier
-------------------+-------------+----------
ct_key | float8 | not null
ipd_date | date | not null
day | varchar(10) | not null
ipd_sessions | float8 |
ipd_bill_amt | float8 |
ipd_byte_qty | float8 |
gr_ipd_sessions | float8 |
gr_bill_amt_total | float8 |
gr_byte_qty_total | float8 |

thanks,

mikeo