diff --git a/example/save.js b/example/save.js
index 9524c6c..1270eea 100644
--- a/example/save.js
+++ b/example/save.js
@@ -1,6 +1,6 @@
 import * as THREE from 'three';
 import { defaultSettings, Slicer } from 'src/index.js';
-import { saveAs } from 'file-saver';
+import fileSaver from 'file-saver';
 
 const settings = {
   ...defaultSettings.base,
@@ -12,16 +12,15 @@ const settings = {
 const jsonLoader = new THREE.JSONLoader();
 jsonLoader.load('models/airplane.json', async geometry => {
   geometry.applyMatrix(new THREE.Matrix4().makeRotationX(Math.PI / -2));
-  geometry.applyMatrix(new THREE.Matrix4().setPosition(new THREE.Vector3(50, -0.1, 50)));
+  geometry.applyMatrix(new THREE.Matrix4().setPosition(new THREE.Vector3(50, -0.0, 50)));
   geometry.computeFaceNormals();
 
   const slicer = new Slicer().setGeometry(geometry);
-  const gcode = await slicer.slice(settings)
-    .progress(({ progress: { done, total, action } }) => {
-      const percentage = `${(done / total * 100).toFixed()}%`
-      document.write(`<p>${action}, ${percentage}</p>`);
-    });
+  const gcode = await slicer.slice(settings, ({ progress: { done, total, action } }) => {
+    const percentage = `${(done / total * 100).toFixed()}%`
+    document.write(`<p>${action}, ${percentage}</p>`);
+  });
 
   const file = new File([gcode], 'gcode.gcode', { type: 'text/plain' });
-  saveAs(file);
+  fileSaver.saveAs(file);
 });
diff --git a/src/Slicer.js b/src/Slicer.js
index c80959b..c4d9d34 100644
--- a/src/Slicer.js
+++ b/src/Slicer.js
@@ -26,10 +26,14 @@ export default class {
     return this;
   }
   sliceSync(settings, onProgress) {
+    if (typeof this.geometry === 'undefined') {
+      throw new Error('Geometry is not set, use Slicer.setGeometry or Slicer.setMesh first');
+    }
+
     return slice(this.geometry, settings, onProgress);
   }
   slice(settings, onProgress) {
-    if (!this.geometry) {
+    if (typeof this.geometry === 'undefined') {
       throw new Error('Geometry is not set, use Slicer.setGeometry or Slicer.setMesh first');
     }
 
@@ -48,7 +52,9 @@ export default class {
             break;
           }
           case 'PROGRESS': {
-            onProgress(data);
+            if (typeof onProgress !== 'undefined') {
+              onProgress(data);
+            }
             break;
           }
         }