diff --git a/masque/utils.py b/masque/utils.py
index b1faa5f..bf82cba 100644
--- a/masque/utils.py
+++ b/masque/utils.py
@@ -74,10 +74,12 @@ def remove_colinear_vertices(vertices: numpy.ndarray, closed_path: bool = True)
             closed path. If False, the path is assumed to be open. Default True.
         :return:
         '''
+        vertices = numpy.array(vertices)
+
         # Check for dx0/dy0 == dx1/dy1
 
-        dv = numpy.roll(vertices, 1, axis=0) - vertices #[y0 - yn1, y1-y0, ...]
-        dxdy = dv * numpy.roll(dv, 1, axis=0)[:, ::-1] # [[dx1*dy0, dx1*dy0], ...]
+        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_diff = numpy.abs(numpy.diff(dxdy, axis=1))[:, 0]
         err_mult = 2 * numpy.abs(dxdy).sum(axis=1) + 1e-40