diff --git a/test/test_YoutubeDL.py b/test/test_YoutubeDL.py
index d994682b2..7040cd93a 100644
--- a/test/test_YoutubeDL.py
+++ b/test/test_YoutubeDL.py
@@ -136,6 +136,11 @@ class TestFormatSelection(unittest.TestCase):
         ]
         info_dict = _make_result(formats)
 
+        ydl = YDL({'format': ''}) # no criteria => anything goes
+        ydl.process_ie_result(info_dict.copy())
+        downloaded = ydl.downloaded_info_dicts[0]
+        self.assertEqual(downloaded['format_id'], '35')
+
         ydl = YDL({'format': '20/47'})
         ydl.process_ie_result(info_dict.copy())
         downloaded = ydl.downloaded_info_dicts[0]
diff --git a/youtube_dl/YoutubeDL.py b/youtube_dl/YoutubeDL.py
index dad44435f..624495ebb 100755
--- a/youtube_dl/YoutubeDL.py
+++ b/youtube_dl/YoutubeDL.py
@@ -1293,6 +1293,9 @@ class YoutubeDL(object):
                 '{0}\n\t{1}\n\t{2}^'.format(note, format_spec, ' ' * start[1]))
             return SyntaxError(message)
 
+        if not format_spec:
+            format_spec = 'worst'
+
         PICKFIRST = 'PICKFIRST'
         MERGE = 'MERGE'
         SINGLE = 'SINGLE'