diff --git a/include/picrin/blob.h b/include/picrin/blob.h
index 6c37c2ce..1face9e6 100644
--- a/include/picrin/blob.h
+++ b/include/picrin/blob.h
@@ -8,7 +8,7 @@ struct pic_blob {
 };
 
 #define pic_blob_p(v) (pic_type(v) == PIC_TT_BLOB)
-#define pic_blob_ptr(v) ((struct pic_blob *)(v).u.data)
+#define pic_blob_ptr(v) ((struct pic_blob *)pic_ptr(v))
 
 struct pic_blob *pic_blob_new(pic_state *, char *, int len);
 
diff --git a/include/picrin/port.h b/include/picrin/port.h
index c421bb11..d5b2eaee 100644
--- a/include/picrin/port.h
+++ b/include/picrin/port.h
@@ -23,6 +23,6 @@ struct pic_port {
 };
 
 #define pic_port_p(v) (pic_type(v) == PIC_TT_PORT)
-#define pic_port_ptr(v) ((struct pic_port *)(v).u.data)
+#define pic_port_ptr(v) ((struct pic_port *)pic_ptr(v))
 
 #endif
diff --git a/include/picrin/proc.h b/include/picrin/proc.h
index d1ca2aab..03046abb 100644
--- a/include/picrin/proc.h
+++ b/include/picrin/proc.h
@@ -21,8 +21,8 @@ struct pic_proc {
 #define pic_proc_p(o) (pic_type(o) == PIC_TT_PROC)
 #define pic_env_p(o) (pic_type(o) == PIC_TT_ENV)
 
-#define pic_proc_ptr(o) ((struct pic_proc *)(o).u.data)
-#define pic_env_ptr(o) ((struct pic_env *)(o).u.data)
+#define pic_proc_ptr(o) ((struct pic_proc *)pic_ptr(o))
+#define pic_env_ptr(o) ((struct pic_env *)pic_ptr(o))
 
 #define pic_proc_cfunc_p(o) (pic_proc_ptr(o)->cfunc_p)
 
diff --git a/include/picrin/value.h b/include/picrin/value.h
index df451bd0..4f8caddb 100644
--- a/include/picrin/value.h
+++ b/include/picrin/value.h
@@ -4,7 +4,7 @@
 typedef int pic_sym;
 
 enum pic_vtype {
-  PIC_VTYPE_NIL,
+  PIC_VTYPE_NIL = 1,
   PIC_VTYPE_TRUE,
   PIC_VTYPE_FALSE,
   PIC_VTYPE_UNDEF,
@@ -27,8 +27,8 @@ typedef struct {
   } u;
 } pic_value;
 
-#define pic_vtype(v)				\
-  ((v).type)
+#define pic_ptr(v) ((v).u.data)
+#define pic_vtype(v) ((v).type)
 
 enum pic_tt {
   /* immediate */
@@ -79,10 +79,10 @@ struct pic_proc;
 struct pic_port;
 struct pic_blob;
 
-#define pic_obj_ptr(o) ((struct pic_object *)(o).u.data)
-#define pic_pair_ptr(o) ((struct pic_pair *)(o).u.data)
-#define pic_str_ptr(o) ((struct pic_string *)(o).u.data)
-#define pic_vec_ptr(o) ((struct pic_vector *)(o).u.data)
+#define pic_obj_ptr(o) ((struct pic_object *)pic_ptr(o))
+#define pic_pair_ptr(o) ((struct pic_pair *)pic_ptr(o))
+#define pic_str_ptr(o) ((struct pic_string *)pic_ptr(o))
+#define pic_vec_ptr(o) ((struct pic_vector *)pic_ptr(o))
 
 enum pic_tt pic_type(pic_value);
 const char *pic_type_repr(enum pic_tt);
diff --git a/src/pair.c b/src/pair.c
index 2f522891..231912cc 100644
--- a/src/pair.c
+++ b/src/pair.c
@@ -23,7 +23,7 @@ pic_car(pic_state *pic, pic_value obj)
   if (! pic_pair_p(obj)) {
     pic_error(pic, "pair required");
   }
-  pair = (struct pic_pair *)obj.u.data;
+  pair = pic_pair_ptr(obj);
 
   return pair->car;
 }
@@ -36,7 +36,7 @@ pic_cdr(pic_state *pic, pic_value obj)
   if (! pic_pair_p(obj)) {
     pic_error(pic, "pair required");
   }
-  pair = (struct pic_pair *)obj.u.data;
+  pair = pic_pair_ptr(obj);
 
   return pair->cdr;
 }