diff --git a/tests/test_serializer.py b/tests/test_serializer.py
index 120632572..c077e7f62 100644
--- a/tests/test_serializer.py
+++ b/tests/test_serializer.py
@@ -486,12 +486,16 @@ class TestDefaultOutput:
         assert Serializer({'nested': {'a': '3', 'b': {'c': '4'}}}).data == {'nested': {'a': '3', 'c': '4'}}
 
     def test_default_for_allow_null(self):
-        # allow_null=True should imply default=None
+        """
+        Without an explicit default, allow_null implies default=None when serializing. #5518 #5708
+        """
         class Serializer(serializers.Serializer):
             foo = serializers.CharField()
             bar = serializers.CharField(source='foo.bar', allow_null=True)
+            optional = serializers.CharField(required=False, allow_null=True)
 
-        assert Serializer({'foo': None}).data == {'foo': None, 'bar': None}
+        # allow_null=True should imply default=None when serialising:
+        assert Serializer({'foo': None}).data == {'foo': None, 'bar': None, 'optional': None, }
 
 
 class TestCacheSerializerData: