diff --git a/rest_framework/relations.py b/rest_framework/relations.py
index 0b7c9d864b..d56438ef3f 100644
--- a/rest_framework/relations.py
+++ b/rest_framework/relations.py
@@ -196,7 +196,7 @@ def get_url(self, obj, view_name, request, format):
         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 getattr(obj, 'pk', None) is None:
             return None
 
         lookup_value = getattr(obj, self.lookup_field)