diff --git a/src/kiri/mode/cam/op-area.js b/src/kiri/mode/cam/op-area.js
index aa9440af..13c92ca1 100644
--- a/src/kiri/mode/cam/op-area.js
+++ b/src/kiri/mode/cam/op-area.js
@@ -227,7 +227,7 @@ class OpArea extends CamOp {
             } else
             if (mode === 'trace') {
                 let { tr_type  } = op;
-                let zs = down ? base_util.lerp(zTop, op.thru ? zBottom : area.minZ(), down) : [ bounds.min.z ];
+                let zs = down ? base_util.lerp(zTop, op.thru ? zBottom : Math.max(zBottom, area.minZ()), down) : [ bounds.min.z ];
                 let zroc = 0;
                 let zinc = 1 / zs.length;
                 for (let z of zs) {