diff --git a/geom/plane.hpp b/geom/plane.hpp
index 6713c724..d13aa2e5 100644
--- a/geom/plane.hpp
+++ b/geom/plane.hpp
@@ -81,7 +81,7 @@ namespace util::geom {
     {
         auto d = dot (
             p.coefficients,
-            q.template redim<S+1> (0)
+            q.template redim<S+1> (-1)
         );
 
         return d / norm (normal (p));