diff --git a/extlib/benz/include/picrin/khash.h b/extlib/benz/include/picrin/khash.h
index 3d8d2ed9..e762ae21 100644
--- a/extlib/benz/include/picrin/khash.h
+++ b/extlib/benz/include/picrin/khash.h
@@ -72,6 +72,7 @@ PIC_INLINE khint_t ac_Wang_hash(khint_t key)
   void kh_destroy_##name(pic_state *, kh_##name##_t *h);                 \
   void kh_clear_##name(kh_##name##_t *h);                               \
   khint_t kh_get_##name(const kh_##name##_t *h, khkey_t key);           \
+  khint_t kh_reverse_get_##name(const kh_##name##_t *h, khval_t val);   \
   void kh_resize_##name(pic_state *, kh_##name##_t *h, khint_t new_n_buckets); \
   khint_t kh_put_##name(pic_state *, kh_##name##_t *h, khkey_t key, int *ret); \
   void kh_del_##name(kh_##name##_t *h, khint_t x);
@@ -109,6 +110,17 @@ PIC_INLINE khint_t ac_Wang_hash(khint_t key)
       return ac_iseither(h->flags, i)? h->n_buckets : i;		\
     } else return 0;                                                    \
   }                                                                     \
+  khint_t kh_reverse_get_##name(const kh_##name##_t *h, khval_t val)    \
+  {                                                                     \
+    if (h->n_buckets) {                                                 \
+      khint_t i = 0;                                                    \
+      while (ac_iseither(h->flags, i) || (h->vals[i] != val)) {         \
+        i += 1;                                                         \
+        if (i == h->n_buckets) return -1;                               \
+      }                                                                 \
+      return i;                                                         \
+    } else return -1;                                                   \
+  }                                                                     \
   void kh_resize_##name(pic_state *pic, kh_##name##_t *h, khint_t new_n_buckets) \
   { /* This function uses 0.25*n_buckets bytes of working space instead of [sizeof(key_t+val_t)+.25]*n_buckets. */ \
     int *new_flags = 0;                                                 \
@@ -233,6 +245,7 @@ PIC_INLINE khint_t ac_Wang_hash(khint_t key)
 #define kh_resize(name, h, s) kh_resize_##name(pic, h, s)
 #define kh_put(name, h, k, r) kh_put_##name(pic, h, k, r)
 #define kh_get(name, h, k) kh_get_##name(h, k)
+#define kh_reverse_get(name, h, v) kh_reverse_get_##name(h, v)
 #define kh_del(name, h, k) kh_del_##name(h, k)
 
 #define kh_exist(h, x) (!ac_iseither((h)->flags, (x)))
diff --git a/extlib/benz/include/picrin/reg.h b/extlib/benz/include/picrin/reg.h
index c64c548f..73ebf069 100644
--- a/extlib/benz/include/picrin/reg.h
+++ b/extlib/benz/include/picrin/reg.h
@@ -26,6 +26,7 @@ pic_value pic_reg_ref(pic_state *, struct pic_reg *, void *);
 void pic_reg_set(pic_state *, struct pic_reg *, void *, pic_value);
 void pic_reg_del(pic_state *, struct pic_reg *, void *);
 bool pic_reg_has(pic_state *, struct pic_reg *, void *);
+void *pic_reg_rev_ref(pic_state *, struct pic_reg *, pic_value);
 
 #if defined(__cplusplus)
 }
diff --git a/extlib/benz/reg.c b/extlib/benz/reg.c
index 655f3b08..9d1282d4 100644
--- a/extlib/benz/reg.c
+++ b/extlib/benz/reg.c
@@ -31,6 +31,19 @@ pic_reg_ref(pic_state *pic, struct pic_reg *reg, void *key)
   return kh_val(h, it);
 }
 
+void *
+pic_reg_rev_ref(pic_state *pic, struct pic_reg *reg, pic_value val)
+{
+  khash_t(reg) *h = &reg->hash;
+  khiter_t it;
+
+  it = kh_reverse_get(reg, h, val);
+  if (it == -1) {
+    pic_errorf(pic, "key not found for an element: ~s", val);
+  }
+  return kh_key(h, it);
+}
+
 void
 pic_reg_set(pic_state PIC_UNUSED(*pic), struct pic_reg *reg, void *key, pic_value val)
 {
diff --git a/extlib/benz/vm.c b/extlib/benz/vm.c
index 3e383bef..3e73578d 100644
--- a/extlib/benz/vm.c
+++ b/extlib/benz/vm.c
@@ -198,9 +198,9 @@ vm_gref(pic_state *pic, struct pic_box *slot, pic_sym *uid)
 {
   if (pic_invalid_p(slot->value)) {
     if (uid == NULL) {
-      uid = pic_intern(pic, "unknown"); /* FIXME */
+      uid = pic_reg_rev_ref(pic, pic->globals, pic_obj_value(slot));
     }
-    pic_errorf(pic, "uninitialized global variable: ~a", pic_obj_value(uid));
+    pic_errorf(pic, "uninitialized global variable: %s", pic_symbol_name(pic, uid));
   }
   return slot->value;
 }