diff --git a/extlib/benz/blob.c b/extlib/benz/blob.c
index bff9d67b..88d15617 100644
--- a/extlib/benz/blob.c
+++ b/extlib/benz/blob.c
@@ -53,7 +53,7 @@ pic_blob_bytevector(pic_state *pic)
   data = pic_blob(pic, blob, NULL);
 
   for (i = 0; i < argc; ++i) {
-    pic_assert_type(pic, argv[i], int);
+    TYPE_CHECK(pic, argv[i], int);
 
     if (pic_int(pic, argv[i]) < 0 || pic_int(pic, argv[i]) > 255) {
       pic_error(pic, "byte out of range", 0);
@@ -181,7 +181,7 @@ pic_blob_bytevector_append(pic_state *pic)
 
   len = 0;
   for (i = 0; i < argc; ++i) {
-    pic_assert_type(pic, argv[i], blob);
+    TYPE_CHECK(pic, argv[i], blob);
     pic_blob(pic, argv[i], &l);
     len += l;
   }
@@ -213,7 +213,7 @@ pic_blob_list_to_bytevector(pic_state *pic)
   data = pic_blob(pic, blob, NULL);
 
   pic_for_each (e, list, it) {
-    pic_assert_type(pic, e, int);
+    TYPE_CHECK(pic, e, int);
 
     if (pic_int(pic, e) < 0 || pic_int(pic, e) > 255)
       pic_error(pic, "byte out of range", 0);
diff --git a/extlib/benz/char.c b/extlib/benz/char.c
index 7f9d7666..220b47e7 100644
--- a/extlib/benz/char.c
+++ b/extlib/benz/char.c
@@ -4,6 +4,7 @@
 
 #include "picrin.h"
 #include "picrin/extra.h"
+#include "picrin/private/object.h"
 
 static pic_value
 pic_char_char_p(pic_state *pic)
@@ -54,7 +55,7 @@ pic_char_integer_to_char(pic_state *pic)
                                                         \
     for (i = 0; i < argc; ++i) {                        \
       c = d;                                            \
-      pic_assert_type(pic, argv[i], char);              \
+      TYPE_CHECK(pic, argv[i], char);                   \
       d = pic_char(pic, argv[i]);                       \
                                                         \
       if (! (c op d))					\
diff --git a/extlib/benz/dict.c b/extlib/benz/dict.c
index 34f40de7..29943407 100644
--- a/extlib/benz/dict.c
+++ b/extlib/benz/dict.c
@@ -105,7 +105,7 @@ pic_dict_dictionary(pic_state *pic)
   dict = pic_make_dict(pic);
 
   for (i = 0; i < argc; i += 2) {
-    pic_assert_type(pic, argv[i], sym);
+    TYPE_CHECK(pic, argv[i], sym);
     pic_dict_set(pic, dict, argv[i], argv[i+1]);
   }
 
@@ -217,7 +217,7 @@ pic_dict_alist_to_dictionary(pic_state *pic)
   dict = pic_make_dict(pic);
 
   pic_for_each (e, pic_reverse(pic, alist), it) {
-    pic_assert_type(pic, pic_car(pic, e), sym);
+    TYPE_CHECK(pic, pic_car(pic, e), sym);
     pic_dict_set(pic, dict, pic_car(pic, e), pic_cdr(pic, e));
   }
 
@@ -250,7 +250,7 @@ pic_dict_plist_to_dictionary(pic_state *pic)
   dict = pic_make_dict(pic);
 
   for (e = pic_reverse(pic, plist); ! pic_nil_p(pic, e); e = pic_cddr(pic, e)) {
-    pic_assert_type(pic, pic_cadr(pic, e), sym);
+    TYPE_CHECK(pic, pic_cadr(pic, e), sym);
     pic_dict_set(pic, dict, pic_cadr(pic, e), pic_car(pic, e));
   }
 
diff --git a/extlib/benz/error.c b/extlib/benz/error.c
index 1e902b0b..ee3fc0a9 100644
--- a/extlib/benz/error.c
+++ b/extlib/benz/error.c
@@ -257,7 +257,7 @@ pic_error_error_object_message(pic_state *pic)
 
   pic_get_args(pic, "o", &e);
 
-  pic_assert_type(pic, e, error);
+  TYPE_CHECK(pic, e, error);
 
   return pic_obj_value(pic_error_ptr(pic, e)->msg);
 }
@@ -269,7 +269,7 @@ pic_error_error_object_irritants(pic_state *pic)
 
   pic_get_args(pic, "o", &e);
 
-  pic_assert_type(pic, e, error);
+  TYPE_CHECK(pic, e, error);
 
   return pic_error_ptr(pic, e)->irrs;
 }
@@ -281,7 +281,7 @@ pic_error_error_object_type(pic_state *pic)
 
   pic_get_args(pic, "o", &e);
 
-  pic_assert_type(pic, e, error);
+  TYPE_CHECK(pic, e, error);
 
   return pic_obj_value(pic_error_ptr(pic, e)->type);
 }
diff --git a/extlib/benz/include/picrin/extra.h b/extlib/benz/include/picrin/extra.h
index ffe1c3d7..40ef36c2 100644
--- a/extlib/benz/include/picrin/extra.h
+++ b/extlib/benz/include/picrin/extra.h
@@ -59,11 +59,6 @@ xFILE *xfopen_null(pic_state *, const char *mode);
 #define pic_push(pic, item, place) (place = pic_cons(pic, item, place))
 #define pic_pop(pic, place) (place = pic_cdr(pic, place))
 
-#define pic_assert_type(pic, v, type) do {      \
-    if (! pic_##type##_p(pic, v))               \
-      pic_error(pic, #type " required", 1, v);  \
-  } while (0)
-
 #define pic_void(exec) pic_void_(PIC_GENSYM(ai), exec)
 #define pic_void_(ai,exec) do {                 \
     size_t ai = pic_enter(pic);                 \
diff --git a/extlib/benz/include/picrin/private/object.h b/extlib/benz/include/picrin/private/object.h
index 6abe4d63..5088468e 100644
--- a/extlib/benz/include/picrin/private/object.h
+++ b/extlib/benz/include/picrin/private/object.h
@@ -158,6 +158,22 @@ struct object *pic_obj_ptr(pic_value);
 pic_value pic_obj_value(void *ptr);
 struct object *pic_obj_alloc(pic_state *, size_t, int type);
 
+#define TYPENAME_int   "integer"
+#define TYPENAME_blob  "bytevector"
+#define TYPENAME_char  "character"
+#define TYPENAME_sym   "symbol"
+#define TYPENAME_error "error"
+#define TYPENAME_proc  "procedure"
+#define TYPENAME_str   "string"
+#define TYPENAME_id    "identifier"
+#define TYPENAME_env   "environment"
+#define TYPENAME_vec   "vector"
+
+#define TYPE_CHECK(pic, v, type) do {                           \
+    if (! pic_##type##_p(pic, v))                               \
+      pic_error(pic, TYPENAME_##type " required", 1, v);        \
+  } while (0)
+
 #define VALID_INDEX(pic, len, i) do {                                   \
     if (i < 0 || len <= i) pic_error(pic, "index out of range", 1, pic_int_value(pic, i)); \
   } while (0)
diff --git a/extlib/benz/proc.c b/extlib/benz/proc.c
index 93cb3831..a097d3b7 100644
--- a/extlib/benz/proc.c
+++ b/extlib/benz/proc.c
@@ -947,7 +947,7 @@ pic_funcall(pic_state *pic, const char *lib, const char *name, int n, ...)
 
   proc = pic_ref(pic, lib, name);
 
-  pic_assert_type(pic, proc, proc);
+  TYPE_CHECK(pic, proc, proc);
 
   va_start(ap, n);
   r = pic_vcall(pic, proc, n, ap);
diff --git a/extlib/benz/string.c b/extlib/benz/string.c
index eff041ed..0250e24b 100644
--- a/extlib/benz/string.c
+++ b/extlib/benz/string.c
@@ -370,7 +370,7 @@ pic_str_string(pic_state *pic)
   buf = pic_alloca(pic, argc);
 
   for (i = 0; i < argc; ++i) {
-    pic_assert_type(pic, argv[i], char);
+    TYPE_CHECK(pic, argv[i], char);
     buf[i] = pic_char(pic, argv[i]);
   }
 
@@ -565,7 +565,7 @@ pic_str_string_append(pic_state *pic)
   pic_get_args(pic, "*", &argc, &argv);
 
   for (i = 0; i < argc; ++i) {
-    pic_assert_type(pic, argv[i], str);
+    TYPE_CHECK(pic, argv[i], str);
     str = pic_str_cat(pic, str, argv[i]);
   }
   return str;
@@ -587,7 +587,7 @@ pic_str_string_map(pic_state *pic)
   len = INT_MAX;
   for (i = 0; i < argc; ++i) {
     int l;
-    pic_assert_type(pic, argv[i], str);
+    TYPE_CHECK(pic, argv[i], str);
     l = pic_str_len(pic, argv[i]);
     len = len < l ? len : l;
   }
@@ -602,7 +602,7 @@ pic_str_string_map(pic_state *pic)
     vals = pic_reverse(pic, vals);
     val = pic_funcall(pic, "picrin.base", "apply", 2, proc, vals);
 
-    pic_assert_type(pic, val, char);
+    TYPE_CHECK(pic, val, char);
 
     buf[i] = pic_char(pic, val);
   }
@@ -624,7 +624,7 @@ pic_str_string_for_each(pic_state *pic)
   len = INT_MAX;
   for (i = 0; i < argc; ++i) {
     int l;
-    pic_assert_type(pic, argv[i], str);
+    TYPE_CHECK(pic, argv[i], str);
     l = pic_str_len(pic, argv[i]);
     len = len < l ? len : l;
   }
@@ -653,7 +653,7 @@ pic_str_list_to_string(pic_state *pic)
 
   i = 0;
   pic_for_each (e, list, it) {
-    pic_assert_type(pic, e, char);
+    TYPE_CHECK(pic, e, char);
 
     buf[i++] = pic_char(pic, e);
   }
diff --git a/extlib/benz/symbol.c b/extlib/benz/symbol.c
index f67f7593..89ddd096 100644
--- a/extlib/benz/symbol.c
+++ b/extlib/benz/symbol.c
@@ -130,8 +130,8 @@ pic_symbol_make_identifier(pic_state *pic)
 
   pic_get_args(pic, "oo", &id, &env);
 
-  pic_assert_type(pic, id, id);
-  pic_assert_type(pic, env, env);
+  TYPE_CHECK(pic, id, id);
+  TYPE_CHECK(pic, env, env);
 
   return pic_make_identifier(pic, id, env);
 }
@@ -143,7 +143,7 @@ pic_symbol_identifier_base(pic_state *pic)
 
   pic_get_args(pic, "o", &id);
 
-  pic_assert_type(pic, id, id);
+  TYPE_CHECK(pic, id, id);
 
   if (pic_sym_p(pic, id)) {
     pic_error(pic, "non-symbol identifier required", 1, id);
@@ -159,7 +159,7 @@ pic_symbol_identifier_environment(pic_state *pic)
 
   pic_get_args(pic, "o", &id);
 
-  pic_assert_type(pic, id, id);
+  TYPE_CHECK(pic, id, id);
 
   if (pic_sym_p(pic, id)) {
     pic_error(pic, "non-symbol identifier required", 1, id);
diff --git a/extlib/benz/vector.c b/extlib/benz/vector.c
index 781d138a..351a0bc4 100644
--- a/extlib/benz/vector.c
+++ b/extlib/benz/vector.c
@@ -180,7 +180,7 @@ pic_vec_vector_append(pic_state *pic)
 
   len = 0;
   for (i = 0; i < argc; ++i) {
-    pic_assert_type(pic, argv[i], vec);
+    TYPE_CHECK(pic, argv[i], vec);
     len += pic_vec_len(pic, argv[i]);
   }
 
@@ -237,7 +237,7 @@ pic_vec_vector_map(pic_state *pic)
   len = INT_MAX;
   for (i = 0; i < argc; ++i) {
     int l;
-    pic_assert_type(pic, argv[i], vec);
+    TYPE_CHECK(pic, argv[i], vec);
     l = pic_vec_len(pic, argv[i]);
     len = len < l ? len : l;
   }
@@ -271,7 +271,7 @@ pic_vec_vector_for_each(pic_state *pic)
   len = INT_MAX;
   for (i = 0; i < argc; ++i) {
     int l;
-    pic_assert_type(pic, argv[i], vec);
+    TYPE_CHECK(pic, argv[i], vec);
     l = pic_vec_len(pic, argv[i]);
     len = len < l ? len : l;
   }
@@ -356,7 +356,7 @@ pic_vec_vector_to_string(pic_state *pic)
   for (i = start; i < end; ++i) {
     t = pic_vec_ref(pic, vec, i);
 
-    pic_assert_type(pic, t, char);
+    TYPE_CHECK(pic, t, char);
 
     buf[i - start] = pic_char(pic, t);
   }