--- archive-tar.c	2012-04-26 21:25:49.000000000 +0200
+++ archive-tar.c	2012-06-13 16:43:59.220945967 +0200
@@ -104,11 +104,11 @@ static unsigned int ustar_header_chksum(
 	char *p = (char *)header;
 	unsigned int chksum = 0;
 	while (p < header->chksum)
-		chksum += *p++;
+		chksum += (unsigned char)*p++;
 	chksum += sizeof(header->chksum) * ' ';
 	p += sizeof(header->chksum);
 	while (p < (char *)header + sizeof(struct ustar_header))
-		chksum += *p++;
+		chksum += (unsigned char)*p++;
 	return chksum;
 }