C trigger problem

I am trying to write a C trigger. Essentially

    TriggerData *in = (TriggerData *) fcinfo->context;
    HeapTupleHeader tuple=in->tg_trigtuple->t_data;
    Datum datum;    
    datum = GetAttributeByName(tuple, "unit_id", &isnull);

and that last line fails with

ERROR:  cache lookup failed for type 4664

I haven't seen anything similar in the documentation.. What is wrong / the
correct way of extracting values from the old/new rows in a C trigger fn?

Cheers,

Patrick