Can't create an index on NUMERIC(x,y)

ftc=> create index z_test on z(a);
ERROR: Can't find a default operator class for type 1700.
ftc=> \d z
Table = z
+----------------------------------+----------------------------------+-------+

| Field | Type |
Length|
+----------------------------------+----------------------------------+-------+

| a | numeric |
9.3 |
+----------------------------------+----------------------------------+-------+

As you can see above when I try to create an index on a NUMERIC it can't
find the default operator for type xxx, is there a way to fix this? It
hasn't worked since numerics were revised in (6.0?)...

Any advise would be very appreciated, please cc: secret(at)kearneydev(dot)com
not just the list :)

--David