diff --git a/modules/util/viirs_l1b_l2.py b/modules/util/viirs_l1b_l2.py
index 1b586d97c9e725950bcb5d9ca34b26c781707f94..68fcde56602550e40325c61efe10b17d3c4f4c3f 100644
--- a/modules/util/viirs_l1b_l2.py
+++ b/modules/util/viirs_l1b_l2.py
@@ -201,7 +201,7 @@ def run(data_h5f, param_s, tiles, tile_width=64, kernel_size=9, day_night='ANY')
             return
     data = np.stack(grd_s)
 
-    tile_width + 2 * border
+    tile_width += 2 * border
 
     num_keep_x_tiles = 6
     i_skip = tile_width