--- a/service/src/safe_util.c
+++ b/service/src/safe_util.c
@@ -58,7 +58,7 @@ mhash(const char *str, int len)
 	srand(v);
 
 	for (bp = buf, i = 0; i < MHASHLEN / 2; i++)
-		bp += sprintf(bp, "%02x", str[i % len] ^ (rand() % 256));
+		bp += sprintf(bp, "%02x", (unsigned char) str[i % len] ^ (rand() % 256));
 
 	return buf;
 }