diff --git a/yt_dlp/extractor/common.py b/yt_dlp/extractor/common.py
index b99b7e5ab..1d2c443c0 100644
--- a/yt_dlp/extractor/common.py
+++ b/yt_dlp/extractor/common.py
@@ -2451,7 +2451,7 @@ def _parse_smil_formats_and_subtitles(
                     })
                 continue
 
-            src_url = src if src.startswith('http') else urllib.parse.urljoin(base, src)
+            src_url = src if src.startswith('http') else urllib.parse.urljoin(f'{base}/', src)
             src_url = src_url.strip()
 
             if proto == 'm3u8' or src_ext == 'm3u8':