diff --git a/cmd/ytdl/test_ytdl.py b/cmd/ytdl/test_ytdl.py
new file mode 100644
index 0000000..1cbc349
--- /dev/null
+++ b/cmd/ytdl/test_ytdl.py
@@ -0,0 +1,8 @@
+import ytdl
+import unittest
+
+
+class TestYtdl(unittest.TestCase):
+    def test_resolve(self):
+        self.assertIsNotNone(ytdl._resolve('https://youtube.com/watch?v=ygIUF678y40', {'format': 'video', 'quality': 'low'}))
+        self.assertIsNotNone(ytdl._resolve('https://youtube.com/watch?v=WyaEiO4hyik', {'format': 'audio', 'quality': 'high'}))
diff --git a/cmd/ytdl/ytdl.py b/cmd/ytdl/ytdl.py
index 4020369..e30e549 100644
--- a/cmd/ytdl/ytdl.py
+++ b/cmd/ytdl/ytdl.py
@@ -83,7 +83,10 @@ def _choose_url(info, metadata):
 
     # Sort list by field (width for videos, file size for audio)
     sort_field = 'width' if is_video else 'filesize'
-    ordered = sorted(fmt_list, key=lambda x: x[sort_field] or x['format_id'], reverse=True)
+    # Sometime 'filesize' field can be None
+    if not all(x[sort_field] is not None for x in fmt_list):
+        sort_field = 'format_id'
+    ordered = sorted(fmt_list, key=lambda x: x[sort_field], reverse=True)
 
     # Choose an item depending on quality, better at the beginning
     is_high_quality = metadata['quality'] == 'high'