diff --git a/contrib/common/lib/tool/ImageGrid.py b/contrib/common/lib/tool/ImageGrid.py
index 88389615..3fd3032d 100644
--- a/contrib/common/lib/tool/ImageGrid.py
+++ b/contrib/common/lib/tool/ImageGrid.py
@@ -47,8 +47,9 @@ def __init__(
         self._subimage_size = subimage_size
         self.number_images = number_images
         self.numbering_color = clr.Color.from_generic(numbering_color)
-        self.numbering_outline_color = clr.Color.from_generic(numbering_outline_color)
-
+        self.numbering_outline_color = (
+            None if numbering_outline_color is None else clr.Color.from_generic(numbering_outline_color)
+        )
         self.images: list[Image.Image] = []
         self.add_images(*images)