diff --git a/geom/ray.hpp b/geom/ray.hpp
index cce268e7..e4a276a4 100644
--- a/geom/ray.hpp
+++ b/geom/ray.hpp
@@ -72,8 +72,8 @@ namespace util::geom {
     {
         CHECK_SANITY (r);
 
-        return dot (p.coefficients, r.origin.   template redim<S+1> (1)) /
-               dot (p.coefficients, r.direction.template redim<S+1> (0));
+        return -dot (p.coefficients, r.origin.   template redim<S+1> (1)) /
+                dot (p.coefficients, r.direction.template redim<S+1> (0));
     }