diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index 33da0bad0..e2af08a51 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -479,6 +479,16 @@ class Serializer(BaseSerializer):
 
         return ret
 
+    def check_for_none(self, attribute):
+        # We skip `to_representation` for `None` values so that fields do
+        # not have to explicitly deal with that case.
+        #
+        # For related fields with `use_pk_only_optimization` we need to
+        # resolve the pk value.
+        if isinstance(attribute, PKOnlyObject):
+            return attribute.pk
+        return attribute
+
     def to_representation(self, instance):
         """
         Object instance -> Dict of primitive datatypes.
@@ -492,13 +502,7 @@ class Serializer(BaseSerializer):
             except SkipField:
                 continue
 
-            # We skip `to_representation` for `None` values so that fields do
-            # not have to explicitly deal with that case.
-            #
-            # For related fields with `use_pk_only_optimization` we need to
-            # resolve the pk value.
-            check_for_none = attribute.pk if isinstance(attribute, PKOnlyObject) else attribute
-            if check_for_none is None:
+            if self.check_for_none(attribute) is None:
                 ret[field.field_name] = None
             else:
                 ret[field.field_name] = field.to_representation(attribute)