diff --git a/alf/summary/render.py b/alf/summary/render.py
index 829489d78..4d4ab2ca7 100644
--- a/alf/summary/render.py
+++ b/alf/summary/render.py
@@ -115,7 +115,7 @@ def resize(self, height=None, width=None, interploation=cv2.INTER_NEAREST):
         """
         if width is not None and height is not None:
             self._img = cv2.resize(self._img, dsize=(width, height))
-            return
+            return self
         if width is not None:
             scale = float(width) / self._img.shape[1]
         elif height is not None: