Create table doesn't work in plpgsql

Hi,

can I do some table manipulation in plpgsql?
Look at only the "create table" line and the error message:

create function plural (text) returns text as '  begin   create table tmp (num int4);   return $1 || ''s'';   end;' language 'plpgsql'; 
select plural('test'); 
CREATE
ERROR:  copyObject: don't know how to copy 611

What does the error message mean? Where can I read more about it?

Cheers, Volker