diff --git a/yt_dlp/utils/_utils.py b/yt_dlp/utils/_utils.py
index 89a0d4cff1..9f815de26f 100644
--- a/yt_dlp/utils/_utils.py
+++ b/yt_dlp/utils/_utils.py
@@ -1424,7 +1424,8 @@ def write_string(s, out=None, encoding=None):
         s = re.sub(r'([\r\n]+)', r' \1', s)
 
     enc, buffer = None, out
-    if 'b' in getattr(out, 'mode', ''):
+    # `mode` might be `None` (Ref: https://github.com/yt-dlp/yt-dlp/issues/8816)
+    if 'b' in (getattr(out, 'mode', None) or ''):
         enc = encoding or preferredencoding()
     elif hasattr(out, 'buffer'):
         buffer = out.buffer