diff --git a/archive-tar.c b/archive-tar.c
index dc91c6b50d..0ba3f25cf5 100644
--- a/archive-tar.c
+++ b/archive-tar.c
@@ -139,13 +139,13 @@ static void strbuf_append_ext_header(struct strbuf *sb, const char *keyword,
 
 static unsigned int ustar_header_chksum(const struct ustar_header *header)
 {
-	const char *p = (const char *)header;
+	const unsigned char *p = (const unsigned char *)header;
 	unsigned int chksum = 0;
-	while (p < header->chksum)
+	while (p < (const unsigned char *)header->chksum)
 		chksum += *p++;
 	chksum += sizeof(header->chksum) * ' ';
 	p += sizeof(header->chksum);
-	while (p < (const char *)header + sizeof(struct ustar_header))
+	while (p < (const unsigned char *)header + sizeof(struct ustar_header))
 		chksum += *p++;
 	return chksum;
 }