diff --git a/graphene/types/argument.py b/graphene/types/argument.py
index 71026d45..f4cb3bae 100644
--- a/graphene/types/argument.py
+++ b/graphene/types/argument.py
@@ -1,5 +1,7 @@
 from itertools import chain
 
+from graphql import Undefined
+
 from .dynamic import Dynamic
 from .mountedtype import MountedType
 from .structures import NonNull
@@ -41,7 +43,7 @@ class Argument(MountedType):
     def __init__(
         self,
         type_,
-        default_value=None,
+        default_value=Undefined,
         description=None,
         name=None,
         required=False,
@@ -102,9 +104,7 @@ def to_arguments(args, extra_args=None):
             raise ValueError(f'Unknown argument "{default_name}".')
 
         arg_name = default_name or arg.name
-        assert (
-            arg_name not in arguments
-        ), f'More than one Argument have same name "{arg_name}".'
+        assert arg_name not in arguments, f'More than one Argument have same name "{arg_name}".'
         arguments[arg_name] = arg
 
     return arguments