diff --git a/include/chibi/sexp.h b/include/chibi/sexp.h
index 9aea8f7a..f96be078 100755
--- a/include/chibi/sexp.h
+++ b/include/chibi/sexp.h
@@ -16,6 +16,8 @@ extern "C" {
 
 #if defined(_WIN32) || defined(__MINGW32__)
 #include <windows.h>
+#define sexp_isalpha(x) ((isalpha)(x))
+#define sexp_isxdigit(x) ((isxdigit)(x))
 #else
 #if SEXP_USE_DL
 #include <dlfcn.h>
@@ -25,6 +27,8 @@ extern "C" {
 #include <errno.h>
 #include <fcntl.h>
 #endif
+#define sexp_isalpha(x) (isalpha(x))
+#define sexp_isxdigit(x) (isxdigit(x))
 #endif
 
 #ifdef PLAN9
diff --git a/main.c b/main.c
index e928a4e2..ceb7571f 100644
--- a/main.c
+++ b/main.c
@@ -201,10 +201,10 @@ void run_main (int argc, char **argv) {
       arg = ((argv[i][2] == '\0') ? argv[++i] : argv[i]+2);
       check_nonull_arg('h', arg);
       heap_size = strtoul(arg, &arg, 0);
-      if ((isalpha)(*arg)) heap_size *= multiplier(*arg++);
+      if (sexp_isalpha(*arg)) heap_size *= multiplier(*arg++);
       if (*arg == '/') {
         heap_max_size = strtoul(arg+1, &arg, 0);
-        if ((isalpha)(*arg)) heap_max_size *= multiplier(*arg++);
+        if (sexp_isalpha(*arg)) heap_max_size *= multiplier(*arg++);
       }
       break;
     case 'V':
diff --git a/sexp.c b/sexp.c
index 9ab8e21a..d3361e4f 100755
--- a/sexp.c
+++ b/sexp.c
@@ -1539,7 +1539,7 @@ sexp sexp_read_string (sexp ctx, sexp in) {
       case 't': c = '\t'; break;
       case 'x':
         c = sexp_read_char(ctx, in);
-        if (isxdigit(c)) {
+        if (sexp_isxdigit(c)) {
           c = digit_value(c)*16 + digit_value(sexp_read_char(ctx, in));
         } else {
           sexp_push_char(ctx, c, in); c = 'x';
@@ -1651,7 +1651,7 @@ sexp sexp_read_number (sexp ctx, sexp in, int base) {
     c = sexp_read_char(ctx, in);
   }
 
-  for ( ; isxdigit(c); c=sexp_read_char(ctx, in)) {
+  for ( ; sexp_isxdigit(c); c=sexp_read_char(ctx, in)) {
     digit = digit_value(c);
     if ((digit < 0) || (digit >= base))
       break;
@@ -1919,7 +1919,8 @@ sexp sexp_read_raw (sexp ctx, sexp in) {
         if (sexp_string_length(res) == 1) {
           res = sexp_make_character(c1);
         } else if ((c1 == 'x' || c1 == 'X') &&
-                   (isxdigit)(str[1]) && (isxdigit)(str[2]) && str[3] == '\0') {
+                   sexp_isxdigit(str[1])
+                   && sexp_isxdigit(str[2]) && str[3] == '\0') {
           res = sexp_make_character(16 * digit_value(str[1])
                                     + digit_value(str[2]));
         } else {