diff --git a/ashlar/thumbnail.py b/ashlar/thumbnail.py
index 43600424..1103b8be 100644
--- a/ashlar/thumbnail.py
+++ b/ashlar/thumbnail.py
@@ -41,8 +41,8 @@ def make_thumbnail(reader, channel=0, scale=0.05):
 
 
 def calculate_image_offset(img1, img2, upsample_factor=1):
-    ref = utils.window(utils.whiten(img1, 0))
-    test = utils.window(utils.whiten(img2, 0))
+    ref = utils.window(utils.whiten(img1, 1))
+    test = utils.window(utils.whiten(img2, 1))
     shift = phase_cross_correlation(
         ref,
         test,