parse error in create index

Hi,

could someone, please, explain me the following parse error?

adressen=> \d geburtstage
Table = geburtstage
+----------------------------------+----------------------------------+-------+
| Field | Type |
Length|
+----------------------------------+----------------------------------+-------+
| lfd_nr | int4
| 4 |
| geburtstag | date
| 4 |
+----------------------------------+----------------------------------+-------+
adressen=> create index Monat_Tag on geburtstage (date_part('month',
Geburtstag));
ERROR: parser: parse error at or near "'"
adressen=>

Thanks in advance,

--
Hubert Palme
palme(at)uni-wuppertal(dot)de