diff --git a/sbin/geom/class/part/geom_part.c b/sbin/geom/class/part/geom_part.c
index d21e5eb6b65..7f668d32e9a 100644
--- a/sbin/geom/class/part/geom_part.c
+++ b/sbin/geom/class/part/geom_part.c
@@ -547,7 +547,7 @@ gpart_autofill(struct gctl_req *req)
 	last = (off_t)strtoimax(s, NULL, 0);
 	grade = ~0ULL;
 	a_first = ALIGNUP(first + offset, alignment);
-	last = ALIGNDOWN(last + offset, alignment);
+	last = ALIGNDOWN(last + offset + 1, alignment) - 1;
 	if (a_first < start)
 		a_first = start;
 	while ((pp = find_provider(gp, first)) != NULL) {