diff --git a/rest_framework/relations.py b/rest_framework/relations.py
index 3a966c5bf..00a4a2656 100644
--- a/rest_framework/relations.py
+++ b/rest_framework/relations.py
@@ -196,7 +196,7 @@ class HyperlinkedRelatedField(RelatedField):
         attributes are not configured to correctly match the URL conf.
         """
         # Unsaved objects will not yet have a valid URL.
-        if obj.pk is None:
+        if hasattr(obj, 'pk') and obj.pk is None:
             return None
 
         lookup_value = getattr(obj, self.lookup_field)
@@ -361,7 +361,7 @@ class ManyRelatedField(Field):
 
     def get_attribute(self, instance):
         # Can't have any relationships if not created
-        if not instance.pk:
+        if hasattr(instance, 'pk') and instance.pk is None:
             return []
 
         relationship = get_attribute(instance, self.source_attrs)