diff --git a/include/mruby.h b/include/mruby.h
index 04ef018cb..390d593bc 100644
--- a/include/mruby.h
+++ b/include/mruby.h
@@ -334,8 +334,7 @@ mrb_value mrb_obj_clone(mrb_state *mrb, mrb_value self);
 
 /* need to include <ctype.h> to use these macros */
 #ifndef ISPRINT
-/* #define ISASCII(c) isascii((int)(unsigned char)(c)) */
-#define ISASCII(c) 1
+#define ISASCII(c) (!(((int)(unsigned char)(c)) & ~0x7f))
 #define ISPRINT(c) (ISASCII(c) && isprint((int)(unsigned char)(c)))
 #define ISSPACE(c) (ISASCII(c) && isspace((int)(unsigned char)(c)))
 #define ISUPPER(c) (ISASCII(c) && isupper((int)(unsigned char)(c)))