diff --git a/tools/firmware-utils/src/buffalo-lib.c b/tools/firmware-utils/src/buffalo-lib.c
index add3a7e023..b1d5ede0a2 100644
--- a/tools/firmware-utils/src/buffalo-lib.c
+++ b/tools/firmware-utils/src/buffalo-lib.c
@@ -179,7 +179,7 @@ int bcrypt_buf(unsigned char seed, unsigned char *key, unsigned char *src,
 
 uint32_t buffalo_csum(uint32_t csum, void *buf, unsigned long len)
 {
-	char *p = buf;
+	signed char *p = buf;
 
 	while (len--) {
 		int i;