diff --git a/Tests/test_image_thumbnail.py b/Tests/test_image_thumbnail.py
index 3f33ec5cf18..7b40a67b6d4 100644
--- a/Tests/test_image_thumbnail.py
+++ b/Tests/test_image_thumbnail.py
@@ -55,10 +55,10 @@ def test_aspect():
     assert im.size == (34, 50)  # ratio is 0.68
 
 
-def test_float():
+def test_float(self):
     im = Image.new("L", (128, 128))
     im.thumbnail((99.9, 99.9))
-    assert im.size == (100, 100)
+    assert im.size == (99, 99)
 
 
 def test_no_resize():