diff --git a/interp.c b/interp.c
index c35cec4..76ec667 100644
--- a/interp.c
+++ b/interp.c
@@ -61,7 +61,7 @@ value_t run_interpreter(value_t lambda, value_t argv)
 
   register_state(&state, lambda, argv);
 
-  /* Keep going until something attempts to tail-call END_PROGRAM, the original 'k', indicating completion. */
+  /* Keep going until we reach END_PROGRAM, "called" by "exit" builtin. */
   while (state.lambda.value != END_PROGRAM)
   {
     state.ntransients = 0;
@@ -146,6 +146,8 @@ value_t run_interpreter(value_t lambda, value_t argv)
 static value_t vector_ref(value_t v, fixnum_t idx)
 {
   vector_t *vec = get_vector(v);
+  if (!((idx >= 0) && (idx < vec->size)))
+    fprintf(stderr, "idx=%d, vec->size=%d\n", (int)idx, (int)vec->size);
   release_assert((idx >= 0) && (idx < vec->size));
   return vec->elements[idx];
 }
@@ -160,6 +162,13 @@ static uint8_t byte_string_ref(value_t v, fixnum_t idx)
 static value_t struct_ref(value_t v, fixnum_t idx)
 {
   struct_t *s = get_struct(v);
+  if (!((idx >= 0) && (idx < s->nslots)))
+  {
+    fprintf(stderr, "idx=%d; nslots=%d;\n",
+        (int)idx, (int)s->nslots);
+    fprint_value(stderr, v);
+    fputc('\n', stderr);
+  }
   release_assert((idx >= 0) && (idx < s->nslots));
   return s->slots[idx];
 }
@@ -629,7 +638,7 @@ static void register_state(interp_state_t *state, value_t lambda, value_t argv)
   register_gc_root(&state->kw_args,    NIL);
   register_gc_root(&state->kw_vals,    NIL);
   register_gc_root(&state->ctx,        FALSE_VALUE);
-  register_gc_root(&state->k,          END_PROGRAM);
+  register_gc_root(&state->k,          lookup_builtin(BI_EXIT));
 
   register_gc_root(&state->globals,    UNDEFINED);
   register_gc_root(&state->instances,  UNDEFINED);