PostgreSQL syntax question

The following syntax (which is legal under Oracle) does not parse under 
PostreSQL 6.3.2.

 update table1 set col1=(select col2 from table2 where col3='x') where col4='z';

Is there a variant of the syntax which does work?

Cheers and thanks,
Stephen.

========================================================================
Stephen Davies Consulting	    		                            scldad@sdc.com.au
Adelaide, South Australia.     			                            Voice: 61-8-82728863
Computing & Network solutions.	 	                            Fax: 61-8-82741015