diff --git a/test/test_utils.py b/test/test_utils.py
index 100f117889..6c8571f980 100644
--- a/test/test_utils.py
+++ b/test/test_utils.py
@@ -2370,6 +2370,11 @@ Line 1
         headers4 = HTTPHeaderDict({'ytdl-test': 'data;'})
         self.assertEqual(set(headers4.items()), {('Ytdl-Test', 'data;')})
 
+        # common mistake: strip whitespace from values
+        # https://github.com/yt-dlp/yt-dlp/issues/8729
+        headers5 = HTTPHeaderDict({'ytdl-test': ' data; '})
+        self.assertEqual(set(headers5.items()), {('Ytdl-Test', 'data;')})
+
     def test_extract_basic_auth(self):
         assert extract_basic_auth('http://:foo.bar') == ('http://:foo.bar', None)
         assert extract_basic_auth('http://foo.bar') == ('http://foo.bar', None)
diff --git a/yt_dlp/utils/networking.py b/yt_dlp/utils/networking.py
index ed02500110..4b73252cbd 100644
--- a/yt_dlp/utils/networking.py
+++ b/yt_dlp/utils/networking.py
@@ -67,7 +67,7 @@ class HTTPHeaderDict(collections.UserDict, dict):
     def __setitem__(self, key, value):
         if isinstance(value, bytes):
             value = value.decode('latin-1')
-        super().__setitem__(key.title(), str(value))
+        super().__setitem__(key.title(), str(value).strip())
 
     def __getitem__(self, key):
         return super().__getitem__(key.title())