diff --git a/test/test_utils.py b/test/test_utils.py
index e83c8ea11..fed94a906 100644
--- a/test/test_utils.py
+++ b/test/test_utils.py
@@ -499,6 +499,7 @@ class TestUtil(unittest.TestCase):
     def test_str_to_int(self):
         self.assertEqual(str_to_int('123,456'), 123456)
         self.assertEqual(str_to_int('123.456'), 123456)
+        self.assertEqual(str_to_int(523), 523)
 
     def test_url_basename(self):
         self.assertEqual(url_basename('http://foo.de/'), '')
diff --git a/youtube_dl/utils.py b/youtube_dl/utils.py
index b14603d8a..328f037a8 100644
--- a/youtube_dl/utils.py
+++ b/youtube_dl/utils.py
@@ -3519,8 +3519,8 @@ def str_or_none(v, default=None):
 
 def str_to_int(int_str):
     """ A more relaxed version of int_or_none """
-    if int_str is None:
-        return None
+    if not isinstance(int_str, compat_str):
+        return int_str
     int_str = re.sub(r'[,\.\+]', '', int_str)
     return int(int_str)