diff --git a/src/mode/cam/prepare.js b/src/mode/cam/prepare.js
index a51e8b74..9b7d4d02 100644
--- a/src/mode/cam/prepare.js
+++ b/src/mode/cam/prepare.js
@@ -179,6 +179,9 @@
             let remain = poly.first().z - poly.last().z,
                 points = [],
                 point = poly.first();
+            if (down <= 0) {
+                down = remain;
+            }
             for (;;) {
                 if (remain > down * 2) {
                     points.push(point.clone());