diff --git a/include/pocketpy/common/isascii.h b/include/pocketpy/common/isascii.h
new file mode 100644
index 00000000..f427f602
--- /dev/null
+++ b/include/pocketpy/common/isascii.h
@@ -0,0 +1,3 @@
+#pragma once
+
+#define pkpy_isascii(c) ((unsigned)(c) < 0x80)
diff --git a/src/common/str.c b/src/common/str.c
index 9c3fb5fb..c0deb139 100644
--- a/src/common/str.c
+++ b/src/common/str.c
@@ -1,5 +1,6 @@
 #include "pocketpy/common/str.h"
 #include "pocketpy/common/utils.h"
+#include "pocketpy/common/isascii.h"
 
 #include <assert.h>
 #include <string.h>
@@ -24,7 +25,7 @@ void pkpy_Str__ctor(pkpy_Str *self, const char *data){
 
 static void pkpy_Str__check_ascii(pkpy_Str *self, char *p) {
     for(int i = 0; i < self->size; i++){
-        if(!isascii(p[i])){
+        if(!pkpy_isascii(p[i])){
             self->is_ascii = false;
             break;
         }
diff --git a/src/common/str.cpp b/src/common/str.cpp
index f9f14fab..7be7f549 100644
--- a/src/common/str.cpp
+++ b/src/common/str.cpp
@@ -1,5 +1,6 @@
 #include "pocketpy/common/str.hpp"
 #include "pocketpy/common/gil.hpp"
+#include "pocketpy/common/isascii.h"
 
 #include <cassert>
 #include <ostream>
@@ -15,7 +16,7 @@ Str::Str(pair<char*, int> detached) {
     this->is_sso = false;
     this->_ptr = detached.first;
     for(int i = 0; i < size; i++) {
-        if(!isascii(_ptr[i])) {
+        if(!pkpy_isascii(_ptr[i])) {
             is_ascii = false;
             break;
         }