diff --git a/masque/utils.py b/masque/utils.py
index 6de82cd..9588ee6 100644
--- a/masque/utils.py
+++ b/masque/utils.py
@@ -125,7 +125,7 @@ def remove_colinear_vertices(vertices: numpy.ndarray, closed_path: bool = True)
     # Check for dx0/dy0 == dx1/dy1
 
     dv = numpy.roll(vertices, -1, axis=0) - vertices  # [y1-y0, y2-y1, ...]
-    dxdy = dv * numpy.roll(dv, 1, axis=0)[:, ::-1]    # [[dx0*(dy_-1), (dx_-1)*dy0], dx1*dy0, dy1*dy0]]
+    dxdy = dv * numpy.roll(dv, 1, axis=0)[:, ::-1]    # [[dx0*(dy_-1), (dx_-1)*dy0], dx1*dy0, dy1*dx0]]
 
     dxdy_diff = numpy.abs(numpy.diff(dxdy, axis=1))[:, 0]
     err_mult = 2 * numpy.abs(dxdy).sum(axis=1) + 1e-40