Error creating index

I'm trying to add a unique index to a table.  Notice the error message below:


Table    = ordertrack
+----------------------------------+----------------------------------+------+
|              Field               |              Type                |Length|
+----------------------------------+----------------------------------+------+
| ordernumber                      | int8                             |     8|
| total                            | money                            |     4|
| cookie                           | text                             |   var|
| orderseq                         | int4 not null default nextval('o |     4|
+----------------------------------+----------------------------------+------+
Index:    ordertrack_orderseq_key
cloud9=> create unique index ord1 on ordertrack(ordernumber);
ERROR:  Can't find a default operator class for type 20.
cloud9=> 


Is this telling me that I can't have an index of type int8?  Why not?

FreeBSD 2.2.8, PostgreSQL cvsup'd less than a week ago.

Vince.
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