diff --git a/docker/utils/utils.py b/docker/utils/utils.py
index 3735b941..86db84e4 100644
--- a/docker/utils/utils.py
+++ b/docker/utils/utils.py
@@ -131,10 +131,10 @@ def convert_volume_binds(binds):
 def parse_repository_tag(repo):
     column_index = repo.rfind(':')
     if column_index < 0:
-        return repo, ""
+        return repo, None
     tag = repo[column_index+1:]
     slash_index = tag.find('/')
     if slash_index < 0:
         return repo[:column_index], tag
 
-    return repo, ""
+    return repo, None
diff --git a/tests/utils_test.py b/tests/utils_test.py
index 08b7b386..809336bc 100644
--- a/tests/utils_test.py
+++ b/tests/utils_test.py
@@ -7,15 +7,15 @@ class UtilsTest(unittest.TestCase):
 
     def test_parse_repository_tag(self):
         self.assertEqual(parse_repository_tag("root"),
-                         ("root", ""))
+                         ("root", None))
         self.assertEqual(parse_repository_tag("root:tag"),
                          ("root", "tag"))
         self.assertEqual(parse_repository_tag("user/repo"),
-                         ("user/repo", ""))
+                         ("user/repo", None))
         self.assertEqual(parse_repository_tag("user/repo:tag"),
                          ("user/repo", "tag"))
         self.assertEqual(parse_repository_tag("url:5000/repo"),
-                         ("url:5000/repo", ""))
+                         ("url:5000/repo", None))
         self.assertEqual(parse_repository_tag("url:5000/repo:tag"),
                          ("url:5000/repo", "tag"))