diff --git a/src/kiri/print.js b/src/kiri/print.js
index c8b3fc9f..99a1bd8d 100644
--- a/src/kiri/print.js
+++ b/src/kiri/print.js
@@ -835,6 +835,9 @@
         }
 
         function outputThin(lines) {
+            if (!lines) {
+                return;
+            }
             let points = lines.group(2).map(grp => {
                 let [ p1, p2 ] = grp;
                 return newPoint(