diff --git a/masque/shapes/arc.py b/masque/shapes/arc.py
index f95c4f6..6f948cb 100644
--- a/masque/shapes/arc.py
+++ b/masque/shapes/arc.py
@@ -272,13 +272,16 @@ class Arc(PositionableImpl, Shape):
             arc_lengths, thetas = get_arclens(n_pts, *a_ranges[0 if inner else 1], dr=dr)
 
             keep = [0]
-            removable = (numpy.cumsum(arc_lengths) <= max_arclen)
-            start = 1
+            start = 0
             while start < arc_lengths.size:
-                next_to_keep = start + numpy.where(removable)[0][-1]    # TODO: any chance we haven't sampled finely enough?
+                removable = (numpy.cumsum(arc_lengths[start:]) <= max_arclen)
+                if not removable.any():
+                    next_to_keep = start + 1
+                else:
+                    next_to_keep = start + numpy.where(removable)[0][-1] + 1
                 keep.append(next_to_keep)
-                removable = (numpy.cumsum(arc_lengths[next_to_keep + 1:]) <= max_arclen)
-                start = next_to_keep + 1
+                start = next_to_keep
+
             if keep[-1] != thetas.size - 1:
                 keep.append(thetas.size - 1)