diff -u -p -u -r1.110 heaptuple.c
--- heaptuple.c 14 Jul 2006 14:52:16 -0000      1.110
+++ heaptuple.c 27 Jul 2006 23:37:54 -0000
@@ -1470,8 +1470,13 @@ slot_getsomeattrs(TupleTableSlot *slot,
 bool
 slot_attisnull(TupleTableSlot *slot, int attnum)
 {
-       HeapTuple       tuple = slot->tts_tuple;
-       TupleDesc       tupleDesc = slot->tts_tupleDescriptor;
+       HeapTuple       tuple;
+       TupleDesc       tupleDesc;
+
+       assert(slot != NULL);
+
+       tuple =  slot->tts_tuple;
+       tupleDesc = slot->tts_tupleDescriptor;

        /*
         * system attributes are handled by heap_attisnull