Problem with updateRow()

I'm using PostgreSQL 7.3.1 server, and the pg73jdbc2.jar file from the
website (file dated 2/13/2003 on my machine).

I have in my code a select statement like this:

String sql = "SELECT FILENAME, BOUNDARY FROM " + tableName + "
WHERE NAME = '" + name + "' AND CONTEXT = '" + context + "'";
logger.finest("SQL: " + sql);
ResultSet result = stmt.executeQuery(sql);

Later in my code I have this:

while (result.next())
{
// ...
result.updateString("BOUNDARY", event.getBoundary());
result.updateRow();
updated = true;
}

Here's the error I get:

java.sql.SQLException: No Primary Keys

Here's what the tables look like in psql:

simpletest=# \d lfs_mappings_559
Table "lfs_mappings_559"
Attribute | Type | Modifier
----------------+------------------------+----------
name | character varying(40) | not null
context | character varying(80) | not null
filename | character varying(300) | not null
boundary | character varying(50) |
insertion_time | real | not null
can_purge | character varying(8) |
Index: lfs_mappings_559_pkey

simpletest=# \d lfs_mappings_559_pkey
Index "lfs_mappings_559_pkey"
Attribute | Type
-----------+-----------------------
name | character varying(40)
context | character varying(80)
unique btree (primary key)

simpletest=#

Why is this happening? Are updateable resultSets not supported?