diff --git a/src/PIL/Image.py b/src/PIL/Image.py
index 0e9c4722b95..cf1800bee35 100644
--- a/src/PIL/Image.py
+++ b/src/PIL/Image.py
@@ -2236,7 +2236,7 @@ def thumbnail(self, size, resample=BICUBIC, reducing_gap=2.0):
         :returns: None
         """
 
-        x, y = size
+        x, y = map(math.floor, size)
         if x >= self.width and y >= self.height:
             return