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?