diff --git a/lib/libc/memchr.c b/lib/libc/memchr.c
index 2eba47c..7bd3a7e 100644
--- a/lib/libc/memchr.c
+++ b/lib/libc/memchr.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2013-2018, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2013-2019, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -8,10 +8,10 @@
 
 void *memchr(const void *src, int c, size_t len)
 {
-	const char *s = src;
+	const unsigned char *s = src;
 
 	while (len--) {
-		if (*s == c)
+		if (*s == (unsigned char)c)
 			return (void *) s;
 		s++;
 	}