diff --git a/contrib/10.callcc/callcc.c b/contrib/10.callcc/callcc.c
index 80905d8e..b3d9a96c 100644
--- a/contrib/10.callcc/callcc.c
+++ b/contrib/10.callcc/callcc.c
@@ -252,7 +252,7 @@ pic_callcc(pic_state *pic, struct pic_proc *proc)
     pic_value args[1];
 
     /* save the continuation object in proc */
-    c = pic_lambda(pic, cont_call, 1, pic_obj_value(pic_data_value(pic, cont, &cont_type)));
+    c = pic_lambda(pic, cont_call, 1, pic_data_value(pic, cont, &cont_type));
 
     args[0] = pic_obj_value(c);
     return pic_applyk(pic, proc, 1, args);
diff --git a/contrib/30.regexp/src/regexp.c b/contrib/30.regexp/src/regexp.c
index dd611975..130f6ff2 100644
--- a/contrib/30.regexp/src/regexp.c
+++ b/contrib/30.regexp/src/regexp.c
@@ -62,7 +62,7 @@ pic_regexp_regexp(pic_state *pic)
     pic_errorf(pic, "regexp compilation error: %s", errbuf);
   }
 
-  return pic_obj_value(pic_data_value(pic, reg, &regexp_type));
+  return pic_data_value(pic, reg, &regexp_type);
 }
 
 static pic_value
diff --git a/contrib/40.srfi/src/106.c b/contrib/40.srfi/src/106.c
index a52d161d..b82c0f6f 100644
--- a/contrib/40.srfi/src/106.c
+++ b/contrib/40.srfi/src/106.c
@@ -141,7 +141,7 @@ pic_socket_make_socket(pic_state *pic)
     pic_errorf(pic, "%s", strerror(errno));
   }
 
-  return pic_obj_value(pic_data_value(pic, sock, &socket_type));
+  return pic_data_value(pic, sock, &socket_type);
 }
 
 static pic_value
@@ -179,7 +179,7 @@ pic_socket_socket_accept(pic_state *pic)
 
   new_sock = pic_malloc(pic, sizeof(struct pic_socket_t));
   new_sock->fd = fd;
-  return pic_obj_value(pic_data_value(pic, new_sock, &socket_type));
+  return pic_data_value(pic, new_sock, &socket_type);
 }
 
 static pic_value
diff --git a/docs/capi.rst b/docs/capi.rst
index 6e701fb9..03b5cc50 100644
--- a/docs/capi.rst
+++ b/docs/capi.rst
@@ -83,15 +83,12 @@ When you use dynamic memory allocation inside C APIs, you must be caseful about
   pic_create_foo(pic_state *pic)
   {
     struct foo *f;
-    struct pic_data *dat;
 
     pic_get_args(pic, ""); // no args here
 
     f = create_foo();
 
-    data = pic_data_value(pic, md, &foo_type);
-
-    return pic_obj_value(data);
+    return pic_data_value(pic, md, &foo_type);
   }
 
   void
diff --git a/extlib/benz/bool.c b/extlib/benz/bool.c
index 3e474e55..826c4626 100644
--- a/extlib/benz/bool.c
+++ b/extlib/benz/bool.c
@@ -172,7 +172,7 @@ internal_equal_p(pic_state *pic, pic_value x, pic_value y, int depth, khash_t(m)
     return true;
   }
   case PIC_TYPE_DATA: {
-    return pic_data_ptr(x)->data == pic_data_ptr(y)->data;
+    return pic_data(pic, x) == pic_data(pic, y);
   }
   default:
     return false;
diff --git a/extlib/benz/cont.c b/extlib/benz/cont.c
index f47347b6..4007b558 100644
--- a/extlib/benz/cont.c
+++ b/extlib/benz/cont.c
@@ -109,7 +109,7 @@ cont_call(pic_state *pic)
     pic_errorf(pic, "calling dead escape continuation");
   }
 
-  cont = pic_data_ptr(pic_closure_ref(pic, CV_ESCAPE))->data;
+  cont = pic_data(pic, pic_closure_ref(pic, CV_ESCAPE));
   cont->retc = argc;
   cont->retv = argv;
 
@@ -127,7 +127,7 @@ pic_make_cont(pic_state *pic, struct pic_cont *cont)
   struct pic_proc *c;
 
   /* save the escape continuation in proc */
-  c = pic_lambda(pic, cont_call, 2, pic_int_value(pic, cont->id), pic_obj_value(pic_data_value(pic, cont, &cont_type)));
+  c = pic_lambda(pic, cont_call, 2, pic_int_value(pic, cont->id), pic_data_value(pic, cont, &cont_type));
 
   return c;
 }
diff --git a/extlib/benz/data.c b/extlib/benz/data.c
index da8b7d6d..ce7c6530 100644
--- a/extlib/benz/data.c
+++ b/extlib/benz/data.c
@@ -4,18 +4,16 @@
 bool
 pic_data_type_p(pic_state *pic, pic_value obj, const pic_data_type *type)
 {
-  return pic_data_p(pic, obj) && pic_data_ptr(obj)->type == type;
+  return pic_data_p(pic, obj) && pic_data_ptr(pic, obj)->type == type;
 }
 
 void *
-pic_data(pic_state *pic, pic_value data)
+pic_data(pic_state PIC_UNUSED(*pic), pic_value data)
 {
-  pic_assert_type(pic, data, data);
-
-  return pic_data_ptr(data)->data;
+  return pic_data_ptr(pic, data)->data;
 }
 
-struct pic_data *
+pic_value
 pic_data_value(pic_state *pic, void *userdata, const pic_data_type *type)
 {
   struct pic_data *data;
@@ -24,5 +22,5 @@ pic_data_value(pic_state *pic, void *userdata, const pic_data_type *type)
   data->type = type;
   data->data = userdata;
 
-  return data;
+  return pic_obj_value(data);
 }
diff --git a/extlib/benz/gc.c b/extlib/benz/gc.c
index 2a7a29a2..8baa2239 100644
--- a/extlib/benz/gc.c
+++ b/extlib/benz/gc.c
@@ -704,7 +704,7 @@ pic_alloca(pic_state *pic, size_t n)
   static const pic_data_type t = { "pic_alloca", pic_free, 0 };
 
   /* TODO: optimize */
-  return pic_data_value(pic, pic_malloc(pic, n), &t)->data;
+  return pic_data(pic, pic_data_value(pic, pic_malloc(pic, n), &t));
 }
 
 struct pic_object *
diff --git a/extlib/benz/include/picrin.h b/extlib/benz/include/picrin.h
index 9e796dc3..45130110 100644
--- a/extlib/benz/include/picrin.h
+++ b/extlib/benz/include/picrin.h
@@ -58,7 +58,6 @@ struct pic_proc;
 struct pic_port;
 struct pic_error;
 struct pic_env;
-struct pic_data;
 
 typedef struct pic_symbol pic_sym;
 typedef struct pic_id pic_id;
@@ -125,7 +124,7 @@ PIC_INLINE char pic_char(pic_state *, pic_value c);
 #define pic_bool(pic,b) (! pic_false_p(pic, b))
 const char *pic_str(pic_state *, struct pic_string *);
 unsigned char *pic_blob(pic_state *, pic_value blob, int *len);
-void *pic_data(pic_state *, pic_value str);
+void *pic_data(pic_state *, pic_value data);
 
 typedef struct {
   const char *type_name;
@@ -147,7 +146,7 @@ struct pic_string *pic_str_value(pic_state *, const char *str, int len);
 struct pic_string *pic_strf_value(pic_state *, const char *fmt, ...);
 struct pic_string *pic_vstrf_value(pic_state *, const char *fmt, va_list ap);
 pic_value pic_blob_value(pic_state *, const unsigned char *buf, int len);
-struct pic_data *pic_data_value(pic_state *, void *ptr, const pic_data_type *type);
+pic_value pic_data_value(pic_state *, void *ptr, const pic_data_type *type);
 
 enum {
   PIC_TYPE_INVALID = 1,
diff --git a/extlib/benz/include/picrin/object.h b/extlib/benz/include/picrin/object.h
index ff3cb20d..1c1ad585 100644
--- a/extlib/benz/include/picrin/object.h
+++ b/extlib/benz/include/picrin/object.h
@@ -123,12 +123,12 @@ struct pic_port {
 #define pic_pair_ptr(pic, o) ((struct pic_pair *)pic_obj_ptr(o))
 #define pic_blob_ptr(pic, v) ((struct pic_blob *)pic_obj_ptr(v))
 #define pic_vec_ptr(pic, o) ((struct pic_vector *)pic_obj_ptr(o))
-#define pic_dict_ptr(pic, v) ((struct pic_dict *)pic_obj_ptr(v))
+#define pic_dict_ptr(pic, o) ((struct pic_dict *)pic_obj_ptr(o))
+#define pic_data_ptr(pic, o) ((struct pic_data *)pic_obj_ptr(o))
 #define pic_sym_ptr(v) ((pic_sym *)pic_obj_ptr(v))
 #define pic_id_ptr(v) ((pic_id *)pic_obj_ptr(v))
 #define pic_str_ptr(o) ((struct pic_string *)pic_obj_ptr(o))
 #define pic_weak_ptr(v) ((struct pic_weak *)pic_obj_ptr(v))
-#define pic_data_ptr(o) ((struct pic_data *)pic_obj_ptr(o))
 #define pic_context_ptr(o) ((struct pic_context *)pic_obj_ptr(o))
 #define pic_proc_ptr(o) ((struct pic_proc *)pic_obj_ptr(o))
 #define pic_rec_ptr(v) ((struct pic_record *)pic_obj_ptr(v))