diff --git a/src/humanize/filesize.py b/src/humanize/filesize.py
index 9bb1a62..560754c 100644
--- a/src/humanize/filesize.py
+++ b/src/humanize/filesize.py
@@ -101,5 +101,5 @@ def naturalsize(
         if abs_bytes < unit:
             break
 
-    ret: str = format % (base * bytes_ / unit) + s
+    ret: str = format % (base * (bytes_ / unit)) + s
     return ret