diff --git a/include/linux/ctype.h b/include/linux/ctype.h
index 653589e3e30e..f13e4ff6835a 100644
--- a/include/linux/ctype.h
+++ b/include/linux/ctype.h
@@ -22,7 +22,10 @@ extern const unsigned char _ctype[];
 #define isalnum(c)	((__ismask(c)&(_U|_L|_D)) != 0)
 #define isalpha(c)	((__ismask(c)&(_U|_L)) != 0)
 #define iscntrl(c)	((__ismask(c)&(_C)) != 0)
-#define isdigit(c)	((__ismask(c)&(_D)) != 0)
+static inline int isdigit(int c)
+{
+	return '0' <= c && c <= '9';
+}
 #define isgraph(c)	((__ismask(c)&(_P|_U|_L|_D)) != 0)
 #define islower(c)	((__ismask(c)&(_L)) != 0)
 #define isprint(c)	((__ismask(c)&(_P|_U|_L|_D|_SP)) != 0)