diff --git a/youtube_dl/extractor/common.py b/youtube_dl/extractor/common.py
index 611cf95f1..450c7dfd6 100644
--- a/youtube_dl/extractor/common.py
+++ b/youtube_dl/extractor/common.py
@@ -138,6 +138,8 @@ class InfoExtractor(object):
 
     Unless mentioned otherwise, the fields should be Unicode strings.
 
+    Unless mentioned otherwise, None is equivalent to absence of information.
+
     Subclasses of this one should re-define the _real_initialize() and
     _real_extract() methods and define a _VALID_URL regexp.
     Probably, they should also be added to the list of extractors.
diff --git a/youtube_dl/utils.py b/youtube_dl/utils.py
index 2615553c1..2c9081733 100644
--- a/youtube_dl/utils.py
+++ b/youtube_dl/utils.py
@@ -1576,9 +1576,9 @@ US_RATINGS = {
 
 def parse_age_limit(s):
     if s is None:
-        return 0
+        return None
     m = re.match(r'^(?P<age>\d{1,2})\+?$', s)
-    return int(m.group('age')) if m else US_RATINGS.get(s, 0)
+    return int(m.group('age')) if m else US_RATINGS.get(s, None)
 
 
 def strip_jsonp(code):