diff --git a/crates/fj-kernel/src/builder/surface.rs b/crates/fj-kernel/src/builder/surface.rs
index fa94685e7..dc0b8db40 100644
--- a/crates/fj-kernel/src/builder/surface.rs
+++ b/crates/fj-kernel/src/builder/surface.rs
@@ -6,7 +6,7 @@ use crate::{
 };
 
 /// Builder API for [`PartialSurface`]
-pub trait SurfaceBuilder {
+pub trait SurfaceBuilder: Sized {
     /// Build a surface from its two axes
     fn from_axes(u: GlobalPath, v: impl Into<Vector<3>>) -> Self;