diff --git a/youtube_dl/extractor/common.py b/youtube_dl/extractor/common.py
index a9497851c..00645feed 100644
--- a/youtube_dl/extractor/common.py
+++ b/youtube_dl/extractor/common.py
@@ -1443,7 +1443,7 @@ class InfoExtractor(object):
                         url_el = representation.find(_add_ns('BaseURL'))
                         filesize = int_or_none(url_el.attrib.get('{http://youtube.com/yt/2012/10/10}contentLength') if url_el is not None else None)
                         f = {
-                            'format_id': mpd_id or representation_id,
+                            'format_id': '%s-%s' % (mpd_id, representation_id) if mpd_id else representation_id,
                             'url': base_url,
                             'width': int_or_none(representation_attrib.get('width')),
                             'height': int_or_none(representation_attrib.get('height')),