diff --git a/lib/libdisk/write_ia64_disk.c b/lib/libdisk/write_ia64_disk.c
index 0b57fdff5f5..6a57322adb1 100644
--- a/lib/libdisk/write_ia64_disk.c
+++ b/lib/libdisk/write_ia64_disk.c
@@ -123,6 +123,8 @@ write_pmbr(int fd, const struct disk *disk)
 
 	error = 0;
 	nsects = disk->media_size / disk->sector_size;
+	nsects--;	/* The GPT starts at LBA 1 */
+
 	buffer = calloc(disk->sector_size, 1);
 	if (buffer == NULL)
 		return (ENOMEM);