diff --git a/rest_framework/fields.py b/rest_framework/fields.py
index 43fed9aee..48cd74513 100644
--- a/rest_framework/fields.py
+++ b/rest_framework/fields.py
@@ -311,6 +311,10 @@ class Field(object):
         self._creation_counter = Field._creation_counter
         Field._creation_counter += 1
 
+        # If `default` is unset, then use `None` when allow_null is `True`.
+        if default is empty and allow_null:
+            default = None
+
         # If `required` is unset, then use `True` unless a default is provided.
         if required is None:
             required = default is empty and not read_only