diff --git a/src/bin/flat.rs b/src/bin/flat.rs
index c5e22ad..a13bba0 100644
--- a/src/bin/flat.rs
+++ b/src/bin/flat.rs
@@ -322,7 +322,7 @@ impl boundary::Boundary for Outside {
 		if dist <= limit {
 			let pt = base + dist * dir;
 			return match side {
-				1 | 5 => Some((boundary::Id(2), vec2(pt.x, self.space.u(pt.y)), vec2(dir.x, self.space.du(pt.x, dir.y)))),
+				1 | 5 => Some((boundary::Id(2), vec2(pt.x, self.space.u(pt.y)), vec2(dir.x, self.space.du(pt.y, dir.y)))),
 				_ => Some((boundary::Id(1), pt, dir)),
 			};
 		}