diff --git a/panda/src/mathutil/plane_src.I b/panda/src/mathutil/plane_src.I
index 6e5819b9b0..489dda5375 100644
--- a/panda/src/mathutil/plane_src.I
+++ b/panda/src/mathutil/plane_src.I
@@ -188,7 +188,7 @@ flip() {
   _v(0) = -_v(0);
   _v(1) = -_v(1);
   _v(2) = -_v(2);
-  // We don't flip D, that would make the plane flip over the origin.
+  _v(3) = -_v(3);
 }
 
 ////////////////////////////////////////////////////////////////////