diff --git a/lib/kstrtox.c b/lib/kstrtox.c
index 08c1401..d586e6a 100644
--- a/lib/kstrtox.c
+++ b/lib/kstrtox.c
@@ -59,7 +59,7 @@ unsigned int _parse_integer_limit(const char *s, unsigned int base, unsigned lon
 	rv = 0;
 	while (max_chars--) {
 		unsigned int c = *s;
-		unsigned int lc = c | 0x20; /* don't tolower() this line */
+		unsigned int lc = _tolower(c);
 		unsigned int val;
 
 		if ('0' <= c && c <= '9')