diff --git a/avbroot/util.py b/avbroot/util.py
index d8ade89..81a0c82 100644
--- a/avbroot/util.py
+++ b/avbroot/util.py
@@ -142,4 +142,4 @@ def zero_n(f_out, size, buf_size=16384):
     while size:
         to_write = min(len(buf_view), size)
         f_out.write(buf_view[:to_write])
-        size -= n
+        size -= to_write