diff --git a/src/multibyte/btowc.c b/src/multibyte/btowc.c
index 29cb798..a7369a1 100644
--- a/src/multibyte/btowc.c
+++ b/src/multibyte/btowc.c
@@ -1,8 +1,11 @@
 #include <stdio.h>
 #include <wchar.h>
+#include <stdlib.h>
+#include "internal.h"
 
 wint_t btowc(int c)
 {
-	c = (unsigned char)c;
-	return c<128U ? c : EOF;
+	if ((unsigned char)c < 128) return (unsigned char)c;
+	if (MB_CUR_MAX==1 && c!=EOF) return CODEUNIT(c);
+	return WEOF;
 }