diff --git a/yt_dlp/downloader/mhtml.py b/yt_dlp/downloader/mhtml.py
index 3d4f2d7634..9ad9b3f795 100644
--- a/yt_dlp/downloader/mhtml.py
+++ b/yt_dlp/downloader/mhtml.py
@@ -133,7 +133,7 @@ def real_download(self, filename, info_dict):
                 '\r\n'
                 f'--{frag_boundary}\r\n'
                 'Content-Type: text/html; charset=utf-8\r\n'
-                f'Content-Length: {len(stub)}\r\n'
+                f'Content-Length: {len(stub.encode())}\r\n'
                 '\r\n'
                 f'{stub}\r\n').encode())
             extra_state['header_written'] = True