diff --git a/youtube_dl/extractor/common.py b/youtube_dl/extractor/common.py
index f34d36cb0..f7247752e 100644
--- a/youtube_dl/extractor/common.py
+++ b/youtube_dl/extractor/common.py
@@ -69,7 +69,8 @@ class InfoExtractor(object):
                                  download, lower-case.
                                  "http", "https", "rtsp", "rtmp" or so.
                     * preference Order number of this format. If this field is
-                                 present, the formats get sorted by this field.
+                                 present and not None, the formats get sorted
+                                 by this field.
                                  -1 for default (order by other properties),
                                  -2 or smaller for less than default.
     url:            Final video URL.
diff --git a/youtube_dl/extractor/wistia.py b/youtube_dl/extractor/wistia.py
index 584550455..bc31c2e64 100644
--- a/youtube_dl/extractor/wistia.py
+++ b/youtube_dl/extractor/wistia.py
@@ -44,6 +44,7 @@ class WistiaIE(InfoExtractor):
                 'height': a['height'],
                 'filesize': a['size'],
                 'ext': a['ext'],
+                'preference': 1 if atype == 'original' else None,
             })
 
         self._sort_formats(formats)