diff --git a/src/mode/fdm/prepare.js b/src/mode/fdm/prepare.js
index 1e6a129f..6f2981cd 100644
--- a/src/mode/fdm/prepare.js
+++ b/src/mode/fdm/prepare.js
@@ -475,7 +475,7 @@
                 //         })
             });
 
-            update(index / output.length);
+            update(index / levels.length);
         });
 
         return layers;