diff --git a/rtiww/src/vec3.h b/rtiww/src/vec3.h
index 94188b4..c38b002 100644
--- a/rtiww/src/vec3.h
+++ b/rtiww/src/vec3.h
@@ -75,7 +75,7 @@ inline vec3 operator+(const vec3 &u, const vec3 &v) {
 }
 
 inline vec3 operator-(const vec3 &u, const vec3 &v) {
-  return vec3(u.e[0] - v.e[0], u.e[1] - v.e[1], u.e[2] + v.e[2]);
+  return vec3(u.e[0] - v.e[0], u.e[1] - v.e[1], u.e[2] - v.e[2]);
 }
 
 inline vec3 operator*(const vec3 &u, const vec3 &v) {