diff --git a/skimage/io/_plugins/pil_plugin.py b/skimage/io/_plugins/pil_plugin.py
index aca53ebd..4725c1ef 100644
--- a/skimage/io/_plugins/pil_plugin.py
+++ b/skimage/io/_plugins/pil_plugin.py
@@ -74,6 +74,7 @@ def pil_to_ndarray(im, dtype=None, img_num=None):
     while 1:
         frame = im.seek(i)
 
+        # seeking must be done sequentially
         if img_num and not i == img_num:
             i += 1
             continue