diff --git a/masque/shapes/shape.py b/masque/shapes/shape.py
index ee6f19a..efc0859 100644
--- a/masque/shapes/shape.py
+++ b/masque/shapes/shape.py
@@ -139,22 +139,24 @@ class Shape(FlippableImpl, PivotableImpl, RepeatableImpl, AnnotatableImpl,
             for v, v_next in zip(p_verts, numpy.roll(p_verts, -1, axis=0), strict=True):
                 dv = v_next - v
 
-                # Find x-index bounds for the line      # TODO: fix this and err_xmin/xmax for grids smaller than the line / shape
+                # Find x-index bounds for the line
                 gxi_range = numpy.digitize([v[0], v_next[0]], gx)
-                gxi_min = numpy.min(gxi_range - 1).clip(0, len(gx) - 1)
-                gxi_max = numpy.max(gxi_range).clip(0, len(gx))
+                gxi_min = int(numpy.min(gxi_range - 1).clip(0, len(gx) - 1))
+                gxi_max = int(numpy.max(gxi_range).clip(0, len(gx)))
 
-                err_xmin = (min(v[0], v_next[0]) - gx[gxi_min]) / (gx[gxi_min + 1] - gx[gxi_min])
-                err_xmax = (max(v[0], v_next[0]) - gx[gxi_max - 1]) / (gx[gxi_max] - gx[gxi_max - 1])
+                if gxi_min < len(gx) - 1:
+                    err_xmin = (min(v[0], v_next[0]) - gx[gxi_min]) / (gx[gxi_min + 1] - gx[gxi_min])
+                    if err_xmin >= 0.5:
+                        gxi_min += 1
 
-                if err_xmin >= 0.5:
-                    gxi_min += 1
-                if err_xmax >= 0.5:
-                    gxi_max += 1
+                if gxi_max > 0 and gxi_max < len(gx):
+                    err_xmax = (max(v[0], v_next[0]) - gx[gxi_max - 1]) / (gx[gxi_max] - gx[gxi_max - 1])
+                    if err_xmax >= 0.5:
+                        gxi_max += 1
 
                 if abs(dv[0]) < 1e-20:
                     # Vertical line, don't calculate slope
-                    xi = [gxi_min, gxi_max - 1]
+                    xi = [gxi_min, max(gxi_min, gxi_max - 1)]
                     ys = numpy.array([v[1], v_next[1]])
                     yi = numpy.digitize(ys, gy).clip(1, len(gy) - 1)
                     err_y = (ys - gy[yi]) / (gy[yi] - gy[yi - 1])