diff --git a/lib/generator.py b/lib/generator.py
index 8913c6c9..9b1f8bce 100755
--- a/lib/generator.py
+++ b/lib/generator.py
@@ -246,6 +246,9 @@ class Generator():
             with open(os.path.join('builder-hex0', 'builder-hex0-x86-stage2.hex0'),
                       encoding="utf-8") as infile:
                 image_file.write(infile.read().encode())
+
+        # Close first with statement before getting file size.
+        with open(image_file_name, 'ab') as image_file:
             # Pad to next sector
             current_size = os.stat(image_file_name).st_size
             while current_size % 512 != 0: