diff --git a/yt_dlp/YoutubeDL.py b/yt_dlp/YoutubeDL.py
index 708cf2abb2..03bcfbb288 100644
--- a/yt_dlp/YoutubeDL.py
+++ b/yt_dlp/YoutubeDL.py
@@ -2477,6 +2477,15 @@ def sanitize_numeric_fields(info):
         if (info_dict.get('duration') or 0) <= 0 and info_dict.pop('duration', None):
             self.report_warning('"duration" field is negative, there is an error in extractor')
 
+        chapters = info_dict.get('chapters') or []
+        dummy_chapter = {'end_time': 0, 'start_time': info_dict.get('duration')}
+        for prev, current, next_ in zip(
+                (dummy_chapter, *chapters), chapters, (*chapters[1:], dummy_chapter)):
+            if current.get('start_time') is None:
+                current['start_time'] = prev.get('end_time')
+            if not current.get('end_time'):
+                current['end_time'] = next_.get('start_time')
+
         if 'playlist' not in info_dict:
             # It isn't part of a playlist
             info_dict['playlist'] = None
diff --git a/yt_dlp/downloader/hls.py b/yt_dlp/downloader/hls.py
index d93d00f123..1e75c5e9c3 100644
--- a/yt_dlp/downloader/hls.py
+++ b/yt_dlp/downloader/hls.py
@@ -61,12 +61,18 @@ def real_download(self, filename, info_dict):
         s = urlh.read().decode('utf-8', 'ignore')
 
         can_download, message = self.can_download(s, info_dict, self.params.get('allow_unplayable_formats')), None
-        if can_download and not Cryptodome_AES and '#EXT-X-KEY:METHOD=AES-128' in s:
-            if FFmpegFD.available():
+        if can_download:
+            has_ffmpeg = FFmpegFD.available()
+            no_crypto = not Cryptodome_AES and '#EXT-X-KEY:METHOD=AES-128' in s
+            if no_crypto and has_ffmpeg:
                 can_download, message = False, 'The stream has AES-128 encryption and pycryptodomex is not available'
-            else:
+            elif no_crypto:
                 message = ('The stream has AES-128 encryption and neither ffmpeg nor pycryptodomex are available; '
                            'Decryption will be performed natively, but will be extremely slow')
+            elif re.search(r'#EXT-X-MEDIA-SEQUENCE:(?!0$)', s):
+                install_ffmpeg = '' if has_ffmpeg else 'install ffmpeg and '
+                message = ('Live HLS streams are not supported by the native downloader. If this is a livestream, '
+                           f'please {install_ffmpeg}add "--downloader ffmpeg --hls-use-mpegts" to your command')
         if not can_download:
             has_drm = re.search('|'.join([
                 r'#EXT-X-FAXS-CM:',  # Adobe Flash Access
diff --git a/yt_dlp/extractor/cwtv.py b/yt_dlp/extractor/cwtv.py
index 07239f39cf..9b83264ee1 100644
--- a/yt_dlp/extractor/cwtv.py
+++ b/yt_dlp/extractor/cwtv.py
@@ -91,4 +91,5 @@ def _real_extract(self, url):
             'timestamp': parse_iso8601(video_data.get('start_time')),
             'age_limit': parse_age_limit(video_data.get('rating')),
             'ie_key': 'ThePlatform',
+            'thumbnail': video_data.get('large_thumbnail')
         }