Syntax problem with INDEX on expression

The following expression works fine in a query:

sp_dev=# select (xpath('//rms:xsid/text()',udh,array[array['rms','
http://www.example.com']]))[1]::text from test;
xpath
----------------------
0000000125846
0000000125843
0000000125844
0000000125842
0000000125845
0000000125847

(10 rows)

However, when the same expression doesn't seem to work for creating an
index:

sp_dev=# create index test_idx on test
((xpath('//rms:xsid/text()',udh,array[array['rms','http://www.example.com
']]))[1]::text);
ERROR: syntax error at or near "["
LINE 1: ...udh,array[array['rms','http://www.example.com']]))[1]::text)...
^

It looks like it doesn't like the array subscript. What might I be missing?

__________________________________________________________________________________
*Mike Blackwell | Technical Analyst, Distribution Services/Rollout
Management | RR Donnelley*
1750 Wallace Ave | St Charles, IL 60174-3401
Office: 630.313.7818
Mike(dot)Blackwell(at)rrd(dot)com
http://www.rrdonnelley.com

<http://www.rrdonnelley.com/>
* <Mike(dot)Blackwell(at)rrd(dot)com>*