diff --git a/src/assemblicon/lowlevel/artrdr.py b/src/assemblicon/lowlevel/artrdr.py
index 0dfe6cb..d77160a 100644
--- a/src/assemblicon/lowlevel/artrdr.py
+++ b/src/assemblicon/lowlevel/artrdr.py
@@ -182,11 +182,11 @@ def render_thumbnail(art: Artifact, hws: Iterable[int]) -> Iterator[Artifact]:
     
     # check whether given artifact is square rectangle.
     if art.width != art.height:
-        raise ValueError(f'Thumbnail render only accept square rectangle.')
-    # scale artifact up to the maximum resolution hws if it is too small.
+        raise ValueError('Thumbnail render only accept square rectangle.')
+    # check whether given artifact is larger than the maximum resolution hws.
     max_hws = max(hws)
     if art.width < max_hws:
-        art = art.resize((max_hws, max_hws), PIL.Image.Resampling.LANCZOS)
+        raise ValueError('Given artifact is too small for rendering thumbnail.')
     
     # render thumbnail
     for hw in hws: