diff --git a/Algebra/Vector.hs b/Algebra/Vector.hs
index 20d3d73..c0bf19e 100644
--- a/Algebra/Vector.hs
+++ b/Algebra/Vector.hs
@@ -143,7 +143,7 @@ sortedY xs =
 
 
 -- |Apply a function on the coordinates of a point.
-onPT :: ((Double, Double) -> (Double, Double)) -> PT -> PT
+onPT :: (Coord -> Coord) -> PT -> PT
 onPT f = p2 . f . unp2