diff --git a/rtchallenge/src/shapes.rs b/rtchallenge/src/shapes.rs
index bb9bc69..682b24b 100644
--- a/rtchallenge/src/shapes.rs
+++ b/rtchallenge/src/shapes.rs
@@ -47,11 +47,18 @@ impl PartialEq for Geometry {
 #[builder(default)]
 pub struct Shape {
     transform: Matrix4x4,
+    #[builder(private, default = "self.default_inverse_transform()?")]
     inverse_transform: Matrix4x4,
     pub material: Material,
     geometry: Geometry,
 }
 
+impl ShapeBuilder {
+    fn default_inverse_transform(&self) -> Result<Matrix4x4, String> {
+        Ok(self.transform.unwrap_or(Matrix4x4::identity()).inverse())
+    }
+}
+
 impl Default for Shape {
     fn default() -> Shape {
         Shape {