diff --git a/slicer/slice.c b/slicer/slice.c
index 8022edd..167bb07 100644
--- a/slicer/slice.c
+++ b/slicer/slice.c
@@ -79,6 +79,8 @@ static void add(const struct v *a, const struct v *b, const struct v *c)
 fprintf(stderr, "zero point\n");
 		return;
 	}
+	if (c->z < a->z)
+		return;
 	z = get_z(a->z);
 	line = alloc_type(struct line);
 	line->ax = a->x;