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@kearneydev.com
not just the list :)

--David