diff --git a/masque/shapes/polygon.py b/masque/shapes/polygon.py
index fc17f61..c8c3ddd 100644
--- a/masque/shapes/polygon.py
+++ b/masque/shapes/polygon.py
@@ -256,6 +256,11 @@ class Polygon(Shape):
         Returns:
             A Polygon object containing the requested rectangle
         """
+        if sum(int(pp is None) for pp in (xmin, xmax, xctr, lx)) != 2:
+            raise PatternError('Exactly two of xmin, xctr, xmax, lx must be provided!')
+        if sum(int(pp is None) for pp in (ymin, ymax, yctr, ly)) != 2:
+            raise PatternError('Exactly two of ymin, yctr, ymax, ly must be provided!')
+
         if lx is None:
             if xctr is None:
                 assert xmin is not None
diff --git a/masque/utils/vertices.py b/masque/utils/vertices.py
index 23fb601..5fddd52 100644
--- a/masque/utils/vertices.py
+++ b/masque/utils/vertices.py
@@ -18,9 +18,9 @@ def remove_duplicate_vertices(vertices: ArrayLike, closed_path: bool = True) ->
         `vertices` with no consecutive duplicates. This may be a view into the original array.
     """
     vertices = numpy.asarray(vertices)
-    duplicates = (vertices == numpy.roll(vertices, 1, axis=0)).all(axis=1)
+    duplicates = (vertices == numpy.roll(vertices, -1, axis=0)).all(axis=1)
     if not closed_path:
-        duplicates[0] = False
+        duplicates[-1] = False
     return vertices[~duplicates]