Segfaults and Bus Errors

I don't know if anyone can help me, but I figured I'd ask anyway.

Using C, i'm trying to copy from one PlTerm to another. The problem
seems to be that what I'm trying to copy is a structure.

sentence(subject(np(det(the),noun(boy),rel(relpron(who),vp(trans_v(reads),do(np(det(a),noun(book))))),prep_phr(prep(under),np(det(a),noun(tree))))),predicate(vp(trans_v(gives),objects(io(np(det(the),noun(girl))),do(np(det(a),noun(truck)))))))

Copy_Term results in an "Illegal instruction" error that dies on me.
After using Rd_Compound_Check to get the functor and arity, I use
Un_Compound_Check, but only the first half of the structure gets
unified.

subject(np(det(the),noun(boy),rel(relpron(who),vp(trans_v(reads),do(np(det(a),noun(book))))),prep_phr(prep(under),np(det(a),noun(tree)))))

Can anybody help with this? I can provide some code if need be.

Thanks in advance,
CB