diff --git a/wcsaxes/coordinate_helpers.py b/wcsaxes/coordinate_helpers.py
index c4c6490..dc93ec2 100644
--- a/wcsaxes/coordinate_helpers.py
+++ b/wcsaxes/coordinate_helpers.py
@@ -573,12 +573,13 @@ def _update_grid_lines(self):
 
         self.grid_lines = []
         for iw, w in enumerate(tick_world_coordinates_values):
+            subset = slice(iw * n_samples, (iw + 1) * n_samples)
             if self.coord_index == 0:
-                xy_world[iw * n_samples:(iw + 1) * n_samples, 0] = np.repeat(w, n_samples)
-                xy_world[iw * n_samples:(iw + 1) * n_samples, 1] = np.linspace(coord_range[1][0], coord_range[1][1], n_samples)
+                xy_world[subset, 0] = np.repeat(w, n_samples)
+                xy_world[subset, 1] = np.linspace(coord_range[1][0], coord_range[1][1], n_samples)
             else:
-                xy_world[iw * n_samples:(iw + 1) * n_samples, 0] = np.linspace(coord_range[0][0], coord_range[0][1], n_samples)
-                xy_world[iw * n_samples:(iw + 1) * n_samples, 1] = np.repeat(w, n_samples)
+                xy_world[subset, 0] = np.linspace(coord_range[0][0], coord_range[0][1], n_samples)
+                xy_world[subset, 1] = np.repeat(w, n_samples)
 
         # We now convert all the world coordinates to pixel coordinates in a
         # single go rather than doing this in the gridline to path conversion
@@ -591,9 +592,8 @@ def _update_grid_lines(self):
         xy_world_round = self.transform.transform(pixel)
 
         for iw in range(n_coord):
-            self.grid_lines.append(self._get_gridline(xy_world[iw * n_samples:(iw + 1) * n_samples],
-                                                      pixel[iw * n_samples:(iw + 1) * n_samples],
-                                                      xy_world_round[iw * n_samples:(iw + 1) * n_samples]))
+            subset = slice(iw * n_samples, (iw + 1) * n_samples)
+            self.grid_lines.append(self._get_gridline(xy_world[subset], pixel[subset], xy_world_round[subset]))
 
     def _get_gridline(self, xy_world, pixel, xy_world_round):
         if self.coord_type == 'scalar':