unplanned sub-select error?

I have a query:

insert into mtr_reg_v_wt (ropnum, inum, pnum, rquant, value, status,
ddate, fr_proj, to_proj) values (28985,1,1014,1,(select cost from
prd_part where pnum = 1014),'work','2005-Nov-15',50,75);

That used to work fine under 7.1.3 but now gives the error:

ERROR: cannot handle unplanned sub-select

Anyone know what this means? Is there a good reason why this update
should no longer work? Or is this a bug?

Kyle
wyatterp.com