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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