diff --git a/python/sglang/srt/mm_utils.py b/python/sglang/srt/mm_utils.py
index 4fdd5eb51..d444ff0b7 100644
--- a/python/sglang/srt/mm_utils.py
+++ b/python/sglang/srt/mm_utils.py
@@ -163,7 +163,9 @@ def expand2square(pil_img, background_color):
     width, height = pil_img.size
     if width == height:
         return pil_img
-    elif width > height:
+    if pil_img.mode == "L":
+        pil_img = pil_img.convert("RGB")
+    if width > height:
         result = Image.new(pil_img.mode, (width, width), background_color)
         result.paste(pil_img, (0, (width - height) // 2))
         return result