convert_numeric_to_scalar: unsupported type 354210

PostgreSQL 7.3.2:
Here is a piece of a table definition:
CREATE TABLE data
(
id SERIAL PRIMARY KEY,
description TEXT,
ra physreal,
dec physreal,
z physreal,
...);
physreal is the domain of numeric(20,14).

Then I created an index across ra,dec, and z (one index). I really
haven't done anything else to the database. The table has ~290000 rows
and I indexed after inserting the data. Strangely, I can only make
weird selects using these fields. Look:
select dec from data limit 5;
dec
---------------
-1.2378252250
-1.2366515502
-1.2501212847
-1.0732052187
-1.1653486998
(5 rows)

select id from data where dec < 191.456;
id
----
1
2
...this works.

select id from data where dec < 2.0;
ERROR: convert_numeric_to_scalar: unsupported type 354210

This doesn't. The problem persists even when I create individual
indices for each column. It seems this only occurs when I use a number
that is "close" to a value in the column. What's going on and what does
the error mean? Thanks.

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AgentM
agentm(at)cmu(dot)edu