Index: django/db/models/fields/related.py
===================================================================
--- django/db/models/fields/related.py	(revision 2814)
+++ django/db/models/fields/related.py	(working copy)
@@ -239,8 +239,9 @@
             self.join_table = join_table
             self.source_col_name = source_col_name
             self.target_col_name = target_col_name
-            if instance:
-                self._pk_val = self.instance._get_pk_val()
+            self._pk_val = self.instance._get_pk_val()
+            if self._pk_val is None:
+                raise ValueError("%r instance needs to have a primary key value before a many-to-many relationship can be used." % model)
 
         def get_query_set(self):
             return superclass.get_query_set(self).filter(**(self.core_filters))
Index: tests/modeltests/many_to_many/models.py
===================================================================
--- tests/modeltests/many_to_many/models.py	(revision 2814)
+++ tests/modeltests/many_to_many/models.py	(working copy)
@@ -110,8 +110,8 @@
 >>> a2.delete()
 >>> Article.objects.all()
 [Django lets you build Web apps easily]
->>> p1.article_set.all()
-[Django lets you build Web apps easily]
+>>> p2.article_set.all()
+[]
 
 # Adding via the 'other' end of an m2m
 >>> a4 = Article(headline='NASA finds intelligent life on Earth')