Improve error message for bad FK resolution

django/db/models/fields/related.py

@@ -958,6 +958,8 @@ class ForeignObject(RelatedField):
     def resolve_related_fields(self):
         if len(self.from_fields) < 1 or len(self.from_fields) != len(self.to_fields):
             raise ValueError('Foreign Object from and to fields must be the same non-zero length')
+        if isinstance(self.rel.to, basestring):
+            raise ValueError('Related model %r cannot been resolved' % self.rel.to)
         related_fields = []
         for index in range(len(self.from_fields)):
             from_field_name = self.from_fields[index]