cache lookup failed

I am relatively new to postgres, and am struggling with the following,

I am trying to set up a trigger on a table that deletes the foreign key
references in the child table,

I'm executing the delete command in the usual way (assuming already
connected)-
  delete from website where website_id = '$website_id';

which works fine when there is no trigger on the table but once the trigger
is created I get the following error -

fmgr_info: function 47172: cache lookup failed

Has anyone seen a similar problem, any help would be greatly appreciated.

Cheers

Graham