diff --git a/api/onnx_web/chain/tile.py b/api/onnx_web/chain/tile.py
index 6b738556..a6705e5c 100644
--- a/api/onnx_web/chain/tile.py
+++ b/api/onnx_web/chain/tile.py
@@ -2,7 +2,7 @@ import itertools
 from enum import Enum
 from logging import getLogger
 from math import ceil
-from typing import Any, Callable, List, Optional, Protocol, Tuple
+from typing import Any, Callable, List, Optional, Protocol, Tuple, Union
 
 import numpy as np
 from PIL import Image
@@ -138,6 +138,20 @@ def make_tile_mask(
     return mask
 
 
+def get_channels(image: Union[np.ndarray, Image.Image]) -> int:
+    if isinstance(image, np.ndarray):
+        return image.shape[-1]
+
+    if image.mode == "RGBA":
+        return 4
+    elif image.mode == "RGB":
+        return 3
+    elif image.mode == "L":
+        return 1
+
+    raise ValueError("unknown image format")
+
+
 def blend_tiles(
     tiles: List[Tuple[int, int, Image.Image]],
     scale: int,
@@ -151,7 +165,7 @@ def blend_tiles(
         "adjusting tile size from %s to %s based on %s overlap", tile, adj_tile, overlap
     )
 
-    channels = max([4 if tile_image.mode == "RGBA" else 3 for _left, _top, tile_image in tiles])
+    channels = max([get_channels(tile_image) for _left, _top, tile_image in tiles])
     scaled_size = (height * scale, width * scale, channels)
 
     count = np.zeros(scaled_size)