diff --git a/piet-gpu/shader/coarse.comp b/piet-gpu/shader/coarse.comp
index a70318a..f835c2b 100644
--- a/piet-gpu/shader/coarse.comp
+++ b/piet-gpu/shader/coarse.comp
@@ -370,5 +370,7 @@ void main() {
         rd_ix += N_TILE;
         if (rd_ix >= ready_ix && partition_ix >= n_partitions) break;
     }
-    Cmd_End_write(cmd_ref);
+    if (bin_tile_x + tile_x < WIDTH_IN_TILES && bin_tile_y + tile_y < HEIGHT_IN_TILES) {
+        Cmd_End_write(cmd_ref);
+    }
 }
diff --git a/piet-gpu/shader/coarse.spv b/piet-gpu/shader/coarse.spv
index 260db69..e0d89e5 100644
Binary files a/piet-gpu/shader/coarse.spv and b/piet-gpu/shader/coarse.spv differ
diff --git a/piet-gpu/src/lib.rs b/piet-gpu/src/lib.rs
index 0272664..c1ab974 100644
--- a/piet-gpu/src/lib.rs
+++ b/piet-gpu/src/lib.rs
@@ -404,7 +404,7 @@ impl Renderer {
         cmd_buf.dispatch(
             &self.coarse_pipeline,
             &self.coarse_ds,
-            (WIDTH as u32 / 256, HEIGHT as u32 / 256, 1),
+            ((WIDTH as u32 + 255) / 256, (HEIGHT as u32 + 255) / 256, 1),
         );
         cmd_buf.write_timestamp(&query_pool, 6);
         cmd_buf.memory_barrier();